diff --git a/apps/client/src/network/handlers/GameHandler.ts b/apps/client/src/network/handlers/GameHandler.ts
index 3d0bd9c..93ac68e 100644
--- a/apps/client/src/network/handlers/GameHandler.ts
+++ b/apps/client/src/network/handlers/GameHandler.ts
@@ -19,6 +19,7 @@
   NewPlayerPayload,
   PlaceBombPayload,
   PlayerHitPayload,
+  PongPayload,
   RemovePlayerPayload,
   UpdateHurricanesPayload,
   UpdateMapCellsPayload,
@@ -67,9 +68,12 @@
   offPlayerHit: (callback: (payload: PlayerHitPayload) => void) => void;
   onHurricaneHit: (callback: (payload: HurricaneHitPayload) => void) => void;
   offHurricaneHit: (callback: (payload: HurricaneHitPayload) => void) => void;
+  onPong: (callback: (payload: PongPayload) => void) => void;
+  offPong: (callback: (payload: PongPayload) => void) => void;
   sendMove: (x: number, y: number) => void;
   sendPlaceBomb: (payload: PlaceBombPayload) => void;
   sendBombHitReport: (payload: BombHitReportPayload) => void;
+  sendPing: (clientTime: number) => void;
   readyForGame: () => void;
 };
 
@@ -145,6 +149,7 @@
   const hurricaneHitSubscription = createSubscriptionPair(
     protocol.SocketEvents.HURRICANE_HIT,
   );
+  const pongSubscription = createSubscriptionPair(protocol.SocketEvents.PONG);
   const sendMovePayload = createPayloadSender(protocol.SocketEvents.MOVE);
   const sendPlaceBombPayload = createPayloadSender(
     protocol.SocketEvents.PLACE_BOMB,
@@ -152,6 +157,7 @@
   const sendBombHitReportPayload = createPayloadSender(
     protocol.SocketEvents.BOMB_HIT_REPORT,
   );
+  const sendPingPayload = createPayloadSender(protocol.SocketEvents.PING);
   const sendReadyForGame = createVoidSender(
     protocol.SocketEvents.READY_FOR_GAME,
   );
@@ -238,6 +244,12 @@
     offHurricaneHit: (callback) => {
       hurricaneHitSubscription.off(callback);
     },
+    onPong: (callback) => {
+      pongSubscription.on(callback);
+    },
+    offPong: (callback) => {
+      pongSubscription.off(callback);
+    },
     sendMove: (x, y) => {
       const payload: MovePayload = { x, y };
       sendMovePayload(payload);
@@ -248,6 +260,9 @@
     sendBombHitReport: (payload) => {
       sendBombHitReportPayload(payload);
     },
+    sendPing: (clientTime) => {
+      sendPingPayload(clientTime);
+    },
     readyForGame: () => {
       sendReadyForGame();
     },
diff --git a/apps/client/src/scenes/game/GameManager.ts b/apps/client/src/scenes/game/GameManager.ts
index d9d2f1e..97a932c 100644
--- a/apps/client/src/scenes/game/GameManager.ts
+++ b/apps/client/src/scenes/game/GameManager.ts
@@ -30,6 +30,7 @@
   type GameUiState,
 } from "./application/ui/GameUiStateSyncService";
 import { preloadGameStartAssets } from "./application/assets/GameAssetPreloader";
+import { ClockSyncService } from "./application/time/ClockSyncService";
 
 /** GameManager の依存注入オプション型 */
 export type GameManagerDependencies = {
@@ -55,12 +56,15 @@
   private myId: string;
   private container: HTMLDivElement;
   private sessionFacade: GameSessionFacade;
+  private gameActionSender: GameActionSender;
   private runtime: GameSceneRuntime;
   private gameEventFacade: GameEventFacade;
   private combatFacade: CombatLifecycleFacade;
   private lifecycleState: SceneLifecycleState;
   private uiStateSyncService: GameUiStateSyncService;
   private disposableRegistry: DisposableRegistry;
+  private readonly clockSyncService: ClockSyncService;
+  private clockSyncTimeoutId: ReturnType<typeof setTimeout> | null = null;
   private localBombHitCount = 0;
 
   public getStartCountdownSec(): number {
@@ -98,10 +102,15 @@
   ) {
     this.container = container; // 明示的に代入
     this.myId = myId;
-    this.sessionFacade = dependencies.sessionFacade ?? new GameSessionFacade();
+    this.clockSyncService = new ClockSyncService();
+    this.sessionFacade =
+      dependencies.sessionFacade ??
+      new GameSessionFacade({
+        nowMsProvider: () => this.clockSyncService.getSynchronizedNowMs(),
+      });
     this.lifecycleState =
       dependencies.lifecycleState ?? new SceneLifecycleState();
-    const gameActionSender =
+    this.gameActionSender =
       dependencies.gameActionSender ?? new SocketGameActionSender();
     const moveSender = dependencies.moveSender ?? new SocketPlayerMoveSender();
     const sceneFactories = dependencies.sceneFactories;
@@ -122,7 +131,7 @@
       myId: this.myId,
       acquireInputLock: this.lockInput.bind(this),
       onSendBombHitReport: (bombId) => {
-        gameActionSender.sendBombHitReport(bombId);
+        this.gameActionSender.sendBombHitReport(bombId);
       },
       onLocalBombHitCountChanged: (count) => {
         this.localBombHitCount = count;
@@ -135,9 +144,15 @@
       players: this.players,
       myId: this.myId,
       sessionFacade: this.sessionFacade,
-      gameActionSender,
+      gameActionSender: this.gameActionSender,
       moveSender,
       getElapsedMs: () => this.sessionFacade.getElapsedMs(),
+      onPongReceived: (payload) => {
+        this.clockSyncService.updateFromPong(payload);
+      },
+      onGameStartClockHint: (serverNowMs) => {
+        this.clockSyncService.seedFromServerNow(serverNowMs);
+      },
       eventPorts: {
         onGameStarted: this.gameEventFacade.applyGameStarted.bind(
           this.gameEventFacade,
@@ -177,6 +192,12 @@
       lifecycleState: this.lifecycleState,
       app: this.app,
     });
+    this.disposableRegistry.add(() => {
+      this.stopClockSyncLoop();
+    });
+    this.disposableRegistry.add(() => {
+      this.clockSyncService.reset();
+    });
   }
 
   /**
@@ -196,6 +217,7 @@
       return;
     }
 
+    this.startClockSyncLoop();
     this.uiStateSyncService.startTicker();
     this.uiStateSyncService.emitIfChanged(true);
   }
@@ -260,4 +282,28 @@
     this.lifecycleState.markDestroyed();
     this.disposableRegistry.disposeAll();
   }
+
+  /** RTT状況に応じて可変間隔でPING送信を継続する */
+  private startClockSyncLoop(): void {
+    this.stopClockSyncLoop();
+
+    const executeSync = () => {
+      this.gameActionSender.sendPing(Date.now());
+
+      const nextIntervalMs = this.clockSyncService.getRecommendedSyncIntervalMs();
+      this.clockSyncTimeoutId = setTimeout(executeSync, nextIntervalMs);
+    };
+
+    executeSync();
+  }
+
+  /** 進行中のPING送信ループを停止する */
+  private stopClockSyncLoop(): void {
+    if (this.clockSyncTimeoutId === null) {
+      return;
+    }
+
+    clearTimeout(this.clockSyncTimeoutId);
+    this.clockSyncTimeoutId = null;
+  }
 }
diff --git a/apps/client/src/scenes/game/application/GameNetworkSync.ts b/apps/client/src/scenes/game/application/GameNetworkSync.ts
index 864d6c5..2e3132c 100644
--- a/apps/client/src/scenes/game/application/GameNetworkSync.ts
+++ b/apps/client/src/scenes/game/application/GameNetworkSync.ts
@@ -8,6 +8,7 @@
   BombPlacedAckPayload,
   BombPlacedPayload,
   HurricaneHitPayload,
+  PongPayload,
   PlayerHitPayload,
 } from "@repo/shared";
 import { AppearanceResolver } from "./AppearanceResolver";
@@ -30,6 +31,8 @@
   onBombPlacementAcknowledged: (payload: BombPlacedAckPayload) => void;
   onRemotePlayerHit: (payload: PlayerHitPayload) => void;
   onRemoteHurricaneHit: (payload: HurricaneHitPayload) => void;
+  onPongReceived: (payload: PongPayload) => void;
+  onGameStartClockHint: (serverNowMs: number) => void;
 };
 
 /** ゲーム中のネットワークイベント購読と同期処理を管理する */
@@ -57,6 +60,8 @@
     onBombPlacementAcknowledged,
     onRemotePlayerHit,
     onRemoteHurricaneHit,
+    onPongReceived,
+    onGameStartClockHint,
   }: GameNetworkSyncOptions) {
     this.stateApplier = new GameNetworkStateApplier({
       worldContainer,
@@ -70,6 +75,8 @@
       onBombPlacementAcknowledged,
       onRemotePlayerHit,
       onRemoteHurricaneHit,
+      onPongReceived,
+      onGameStartClockHint,
       onDebugLog: this.debugLog,
     });
 
diff --git a/apps/client/src/scenes/game/application/lifecycle/GameSessionFacade.ts b/apps/client/src/scenes/game/application/lifecycle/GameSessionFacade.ts
index 46b8089..9c98b64 100644
--- a/apps/client/src/scenes/game/application/lifecycle/GameSessionFacade.ts
+++ b/apps/client/src/scenes/game/application/lifecycle/GameSessionFacade.ts
@@ -6,12 +6,18 @@
 import { GameTimer } from "@client/scenes/game/application/GameTimer";
 import { InputGate, type JoystickInput } from "./InputGate";
 
+/** GameSessionFacade の初期化入力型 */
+export type GameSessionFacadeOptions = {
+  nowMsProvider?: () => number;
+};
+
 /** ゲーム進行状態と入力可否の窓口を提供する */
 export class GameSessionFacade {
-  private readonly timer = new GameTimer();
+  private readonly timer: GameTimer;
   private readonly inputGate: InputGate;
 
-  constructor() {
+  constructor(options: GameSessionFacadeOptions = {}) {
+    this.timer = new GameTimer(options.nowMsProvider);
     this.inputGate = new InputGate({
       isStartedProvider: () => this.timer.isStarted(),
       isPlayableTimeProvider: () => this.timer.getRemainingTime() > 0,
diff --git a/apps/client/src/scenes/game/application/network/GameActionSender.ts b/apps/client/src/scenes/game/application/network/GameActionSender.ts
index 8c22eb2..5d02df0 100644
--- a/apps/client/src/scenes/game/application/network/GameActionSender.ts
+++ b/apps/client/src/scenes/game/application/network/GameActionSender.ts
@@ -11,6 +11,7 @@
   readyForGame: () => void;
   sendPlaceBomb: (payload: PlaceBombPayload) => void;
   sendBombHitReport: (bombId: string) => void;
+  sendPing: (clientTime: number) => void;
 };
 
 /** ソケット経由でゲーム中送信アクションを実行する実装 */
@@ -29,4 +30,9 @@
   public sendBombHitReport(bombId: string): void {
     socketManager.game.sendBombHitReport({ bombId });
   }
-}
\ No newline at end of file
+
+  /** 時刻同期PINGをサーバーへ送信する */
+  public sendPing(clientTime: number): void {
+    socketManager.game.sendPing(clientTime);
+  }
+}
diff --git a/apps/client/src/scenes/game/application/network/NetworkSubscriptions.ts b/apps/client/src/scenes/game/application/network/NetworkSubscriptions.ts
index 87104b5..c89f5a5 100644
--- a/apps/client/src/scenes/game/application/network/NetworkSubscriptions.ts
+++ b/apps/client/src/scenes/game/application/network/NetworkSubscriptions.ts
@@ -12,6 +12,7 @@
   HurricaneHitPayload,
   NewPlayerPayload,
   PlayerHitPayload,
+  PongPayload,
   RemovePlayerPayload,
   UpdateHurricanesPayload,
   UpdateMapCellsPayload,
@@ -38,6 +39,7 @@
   bombPlacedAck: SocketSubscription;
   playerHit: SocketSubscription;
   hurricaneHit: SocketSubscription;
+  pong: SocketSubscription;
 };
 
 /** 購読辞書生成に必要なハンドラ群 */
@@ -54,6 +56,7 @@
   onBombPlacedAck: (payload: BombPlacedAckPayload) => void;
   onPlayerHit: (payload: PlayerHitPayload) => void;
   onHurricaneHit: (payload: HurricaneHitPayload) => void;
+  onPong: (payload: PongPayload) => void;
 };
 
 type SubscriptionDefinition = {
@@ -154,6 +157,13 @@
       unbind: () => socketManager.game.offHurricaneHit(handlers.onHurricaneHit),
     }),
   },
+  {
+    key: "pong",
+    create: (handlers) => ({
+      bind: () => socketManager.game.onPong(handlers.onPong),
+      unbind: () => socketManager.game.offPong(handlers.onPong),
+    }),
+  },
 ];
 
 /** ソケット購読辞書を生成する */
diff --git a/apps/client/src/scenes/game/application/network/adapters/GameNetworkEventAdapter.ts b/apps/client/src/scenes/game/application/network/adapters/GameNetworkEventAdapter.ts
index c2e3aa9..d2f1640 100644
--- a/apps/client/src/scenes/game/application/network/adapters/GameNetworkEventAdapter.ts
+++ b/apps/client/src/scenes/game/application/network/adapters/GameNetworkEventAdapter.ts
@@ -12,17 +12,14 @@
 } from "@repo/shared";
 
 /** ゲーム開始受信ペイロードから開始時刻を抽出する
- * serverNow を用いてクライアントとサーバーの時計差を補正し，
- * クライアント時計基準の開始時刻を返す
+ * サーバー時刻基準の開始時刻を返す
  */
 export const toGameStartedAt = (payload: GameStartPayload): number | null => {
-  if (!payload || !payload.startTime) {
+  if (!payload || payload.startTime == null) {
     return null;
   }
 
-  // clockOffset > 0: サーバーがクライアントより進んでいる
-  const clockOffset = payload.serverNow - Date.now();
-  return payload.startTime - clockOffset;
+  return payload.startTime;
 };
 
 /** 爆弾設置受信ペイロードを内部ペイロードへ正規化する */
diff --git a/apps/client/src/scenes/game/application/network/handlers/GameNetworkStateApplier.ts b/apps/client/src/scenes/game/application/network/handlers/GameNetworkStateApplier.ts
index 4deec45..69a74d6 100644
--- a/apps/client/src/scenes/game/application/network/handlers/GameNetworkStateApplier.ts
+++ b/apps/client/src/scenes/game/application/network/handlers/GameNetworkStateApplier.ts
@@ -8,6 +8,7 @@
   BombPlacedAckPayload,
   BombPlacedPayload,
   HurricaneHitPayload,
+  PongPayload,
   PlayerHitPayload,
 } from "@repo/shared";
 import { domain } from "@repo/shared";
@@ -40,6 +41,8 @@
   onBombPlacementAcknowledged: (payload: BombPlacedAckPayload) => void;
   onRemotePlayerHit: (payload: PlayerHitPayload) => void;
   onRemoteHurricaneHit: (payload: HurricaneHitPayload) => void;
+  onPongReceived: (payload: PongPayload) => void;
+  onGameStartClockHint: (serverNowMs: number) => void;
   onDebugLog?: (message: string) => void;
 };
 
@@ -51,6 +54,8 @@
   private readonly hurricaneSyncHandler: HurricaneSyncHandler;
   private readonly onGameStarted: (startTime: number) => void;
   private readonly onGameEnded: () => void;
+  private readonly onPongReceived: (payload: PongPayload) => void;
+  private readonly onGameStartClockHint: (serverNowMs: number) => void;
   private readonly onDebugLog: (message: string) => void;
   private readonly receivedEventHandlers: ReceivedGameEventHandlers;
 
@@ -66,6 +71,8 @@
     onBombPlacementAcknowledged,
     onRemotePlayerHit,
     onRemoteHurricaneHit,
+    onPongReceived,
+    onGameStartClockHint,
     onDebugLog,
   }: GameNetworkStateApplierOptions) {
     this.playerSyncHandler = new PlayerSyncHandler({
@@ -84,6 +91,8 @@
     });
     this.onGameStarted = onGameStarted;
     this.onGameEnded = onGameEnded;
+    this.onPongReceived = onPongReceived;
+    this.onGameStartClockHint = onGameStartClockHint;
     this.onDebugLog = onDebugLog ?? (() => undefined);
     this.receivedEventHandlers = this.createReceivedEventHandlers();
   }
@@ -139,6 +148,8 @@
         this.playerSyncHandler.handleNewPlayer(payload);
       },
       onReceivedGameStart: (payload) => {
+        this.onGameStartClockHint(payload.serverNow);
+
         const startTime = toGameStartedAt(payload);
         if (startTime === null) {
           return;
@@ -177,6 +188,9 @@
       onReceivedHurricaneHit: (payload) => {
         this.combatSyncHandler.handleReceivedHurricaneHit(payload);
       },
+      onReceivedPong: (payload) => {
+        this.onPongReceived(payload);
+      },
     };
   }
 }
diff --git a/apps/client/src/scenes/game/application/network/receivers/GameNetworkEventReceiver.ts b/apps/client/src/scenes/game/application/network/receivers/GameNetworkEventReceiver.ts
index f2c782e..8f4d080 100644
--- a/apps/client/src/scenes/game/application/network/receivers/GameNetworkEventReceiver.ts
+++ b/apps/client/src/scenes/game/application/network/receivers/GameNetworkEventReceiver.ts
@@ -11,6 +11,7 @@
   HurricaneHitPayload,
   NewPlayerPayload,
   PlayerHitPayload,
+  PongPayload,
   RemovePlayerPayload,
   UpdateHurricanesPayload,
   UpdateMapCellsPayload,
@@ -35,6 +36,7 @@
   onReceivedBombPlacedAck: (payload: BombPlacedAckPayload) => void;
   onReceivedPlayerHit: (payload: PlayerHitPayload) => void;
   onReceivedHurricaneHit: (payload: HurricaneHitPayload) => void;
+  onReceivedPong: (payload: PongPayload) => void;
 };
 
 /** 受信イベント購読の管理を担当する */
@@ -56,6 +58,7 @@
       onBombPlacedAck: handlers.onReceivedBombPlacedAck,
       onPlayerHit: handlers.onReceivedPlayerHit,
       onHurricaneHit: handlers.onReceivedHurricaneHit,
+      onPong: handlers.onReceivedPong,
     });
   }
 
diff --git a/apps/client/src/scenes/game/application/orchestrators/GameSceneOrchestrator.ts b/apps/client/src/scenes/game/application/orchestrators/GameSceneOrchestrator.ts
index 5520abf..fa616d5 100644
--- a/apps/client/src/scenes/game/application/orchestrators/GameSceneOrchestrator.ts
+++ b/apps/client/src/scenes/game/application/orchestrators/GameSceneOrchestrator.ts
@@ -8,6 +8,7 @@
   BombPlacedAckPayload,
   BombPlacedPayload,
   HurricaneHitPayload,
+  PongPayload,
   PlayerHitPayload,
 } from "@repo/shared";
 import { AppearanceResolver } from "@client/scenes/game/application/AppearanceResolver";
@@ -35,6 +36,8 @@
   onBombPlacementAcknowledged: (payload: BombPlacedAckPayload) => void;
   onRemotePlayerHit: (payload: PlayerHitPayload) => void;
   onRemoteHurricaneHit: (payload: HurricaneHitPayload) => void;
+  onPongReceived: (payload: PongPayload) => void;
+  onGameStartClockHint: (serverNowMs: number) => void;
 };
 
 /** BombManager 生成入力型 */
@@ -88,6 +91,8 @@
   getJoystickInput: () => { x: number; y: number };
   moveSender: MoveSender;
   eventPorts: GameSceneEventPorts;
+  onPongReceived: (payload: PongPayload) => void;
+  onGameStartClockHint: (serverNowMs: number) => void;
   factories?: GameSceneFactoryOptions;
 };
 
@@ -111,6 +116,8 @@
   private readonly getJoystickInput: () => { x: number; y: number };
   private readonly moveSender: MoveSender;
   private readonly eventPorts: GameSceneEventPorts;
+  private readonly onPongReceived: (payload: PongPayload) => void;
+  private readonly onGameStartClockHint: (serverNowMs: number) => void;
   private readonly createNetworkSync: (
     options: CreateNetworkSyncOptions,
   ) => GameNetworkSync;
@@ -130,6 +137,8 @@
     getJoystickInput,
     moveSender,
     eventPorts,
+    onPongReceived,
+    onGameStartClockHint,
     factories,
   }: GameSceneOrchestratorOptions) {
     this.app = app;
@@ -142,6 +151,8 @@
     this.getJoystickInput = getJoystickInput;
     this.moveSender = moveSender;
     this.eventPorts = eventPorts;
+    this.onPongReceived = onPongReceived;
+    this.onGameStartClockHint = onGameStartClockHint;
     this.createNetworkSync =
       factories?.createNetworkSync ??
       ((options) => new GameNetworkSync(options));
@@ -187,6 +198,8 @@
       onBombPlacementAcknowledged: this.eventPorts.onBombPlacementAcknowledged,
       onRemotePlayerHit: this.eventPorts.onRemotePlayerHit,
       onRemoteHurricaneHit: this.eventPorts.onRemoteHurricaneHit,
+      onPongReceived: this.onPongReceived,
+      onGameStartClockHint: this.onGameStartClockHint,
     });
     networkSync.bind();
     return networkSync;
diff --git a/apps/client/src/scenes/game/application/runtime/GameSceneRuntime.ts b/apps/client/src/scenes/game/application/runtime/GameSceneRuntime.ts
index 6fb417e..0aeb79c 100644
--- a/apps/client/src/scenes/game/application/runtime/GameSceneRuntime.ts
+++ b/apps/client/src/scenes/game/application/runtime/GameSceneRuntime.ts
@@ -21,6 +21,7 @@
 import { PlayerRepository } from "@client/scenes/game/entities/player/PlayerRepository";
 import type { GameMapController } from "@client/scenes/game/entities/map/GameMapController";
 import { config } from "@client/config";
+import type { PongPayload } from "@repo/shared";
 
 type RuntimeLifecycleState = "created" | "initialized" | "destroyed";
 
@@ -34,6 +35,8 @@
   moveSender: MoveSender;
   getElapsedMs: () => number;
   eventPorts: GameSceneEventPorts;
+  onPongReceived: (payload: PongPayload) => void;
+  onGameStartClockHint: (serverNowMs: number) => void;
   sceneFactories?: GameSceneFactoryOptions;
 };
 
@@ -48,6 +51,8 @@
   private readonly moveSender: MoveSender;
   private readonly getElapsedMs: () => number;
   private readonly eventPorts: GameSceneEventPorts;
+  private readonly onPongReceived: (payload: PongPayload) => void;
+  private readonly onGameStartClockHint: (serverNowMs: number) => void;
   private readonly sceneFactories?: GameSceneFactoryOptions;
   private readonly playerRepository: PlayerRepository;
   private readonly disposableRegistry = new DisposableRegistry();
@@ -77,6 +82,8 @@
     moveSender,
     getElapsedMs,
     eventPorts,
+    onPongReceived,
+    onGameStartClockHint,
     sceneFactories,
   }: GameSceneRuntimeOptions) {
     this.app = app;
@@ -88,6 +95,8 @@
     this.moveSender = moveSender;
     this.getElapsedMs = getElapsedMs;
     this.eventPorts = eventPorts;
+    this.onPongReceived = onPongReceived;
+    this.onGameStartClockHint = onGameStartClockHint;
     this.sceneFactories = sceneFactories;
     this.playerRepository = new PlayerRepository(this.players);
 
@@ -116,6 +125,8 @@
       getJoystickInput: () => this.joystickInput,
       moveSender: this.moveSender,
       eventPorts: this.eventPorts,
+      onPongReceived: this.onPongReceived,
+      onGameStartClockHint: this.onGameStartClockHint,
       sceneFactories: this.sceneFactories,
     });
 
diff --git a/apps/client/src/scenes/game/application/runtime/GameSceneRuntimeWiring.ts b/apps/client/src/scenes/game/application/runtime/GameSceneRuntimeWiring.ts
index c886e60..3f02d36 100644
--- a/apps/client/src/scenes/game/application/runtime/GameSceneRuntimeWiring.ts
+++ b/apps/client/src/scenes/game/application/runtime/GameSceneRuntimeWiring.ts
@@ -14,6 +14,7 @@
 import type { GamePlayers } from "../game.types";
 import type { MoveSender } from "../network/PlayerMoveSender";
 import type { PlayerRepository } from "@client/scenes/game/entities/player/PlayerRepository";
+import type { PongPayload } from "@repo/shared";
 
 /** Runtime配線処理の入力型 */
 export type GameSceneRuntimeWiringOptions = {
@@ -27,6 +28,8 @@
   getJoystickInput: () => { x: number; y: number };
   moveSender: MoveSender;
   eventPorts: GameSceneEventPorts;
+  onPongReceived: (payload: PongPayload) => void;
+  onGameStartClockHint: (serverNowMs: number) => void;
   sceneFactories?: GameSceneFactoryOptions;
 };
 
@@ -51,6 +54,8 @@
       getJoystickInput: this.options.getJoystickInput,
       moveSender: this.options.moveSender,
       eventPorts: this.options.eventPorts,
+      onPongReceived: this.options.onPongReceived,
+      onGameStartClockHint: this.options.onGameStartClockHint,
       factories: this.options.sceneFactories,
     });
 
diff --git a/apps/client/src/scenes/game/application/time/ClockSyncService.ts b/apps/client/src/scenes/game/application/time/ClockSyncService.ts
new file mode 100644
index 0000000..48daf13
--- /dev/null
+++ b/apps/client/src/scenes/game/application/time/ClockSyncService.ts
@@ -0,0 +1,118 @@
+/**
+ * ClockSyncService
+ * サーバー時刻との差分を平滑化して管理する
+ * PING/PONGのRTTを使い，外れ値を除外して同期精度を安定化する
+ */
+import type { PongPayload } from "@repo/shared";
+
+/** 時刻同期に利用する更新パラメータ */
+export type ClockSyncConfig = {
+  offsetAlpha: number;
+  rttAlpha: number;
+  maxAcceptedRttMs: number;
+  maxAcceptedOffsetJumpMs: number;
+};
+
+/** 時刻同期の既定パラメータ */
+export const DEFAULT_CLOCK_SYNC_CONFIG: ClockSyncConfig = {
+  offsetAlpha: 0.12,
+  rttAlpha: 0.25,
+  maxAcceptedRttMs: 1000,
+  maxAcceptedOffsetJumpMs: 250,
+};
+
+/** サーバー時刻との差分を平滑化して保持する */
+export class ClockSyncService {
+  private readonly config: ClockSyncConfig;
+  private smoothedOffsetMs: number | null = null;
+  private smoothedRttMs: number | null = null;
+
+  constructor(config: Partial<ClockSyncConfig> = {}) {
+    this.config = {
+      ...DEFAULT_CLOCK_SYNC_CONFIG,
+      ...config,
+    };
+  }
+
+  /** 受信した serverNow をもとに差分を初期化する */
+  public seedFromServerNow(serverNowMs: number, receivedAtMs = Date.now()): void {
+    this.smoothedOffsetMs = serverNowMs - receivedAtMs;
+  }
+
+  /** PONGサンプルを取り込み，差分とRTTを更新する */
+  public updateFromPong(payload: PongPayload, receivedAtMs = Date.now()): void {
+    const measuredRttMs = receivedAtMs - payload.clientTime;
+    if (measuredRttMs < 0 || measuredRttMs > this.config.maxAcceptedRttMs) {
+      return;
+    }
+
+    const estimatedOneWayMs = measuredRttMs / 2;
+    const measuredOffsetMs =
+      payload.serverTime - (payload.clientTime + estimatedOneWayMs);
+
+    this.smoothedRttMs = this.smoothValue(
+      this.smoothedRttMs,
+      measuredRttMs,
+      this.config.rttAlpha,
+    );
+
+    if (
+      this.smoothedOffsetMs !== null &&
+      Math.abs(measuredOffsetMs - this.smoothedOffsetMs) >
+        this.config.maxAcceptedOffsetJumpMs
+    ) {
+      return;
+    }
+
+    this.smoothedOffsetMs = this.smoothValue(
+      this.smoothedOffsetMs,
+      measuredOffsetMs,
+      this.config.offsetAlpha,
+    );
+  }
+
+  /** 平滑化済みの時刻差分ミリ秒を返す */
+  public getClockOffsetMs(): number {
+    return this.smoothedOffsetMs ?? 0;
+  }
+
+  /** サーバー時刻基準へ補正した現在時刻ミリ秒を返す */
+  public getSynchronizedNowMs(): number {
+    return Date.now() + this.getClockOffsetMs();
+  }
+
+  /** RTT状況に応じた次回同期推奨間隔ミリ秒を返す */
+  public getRecommendedSyncIntervalMs(): number {
+    if (this.smoothedRttMs === null) {
+      return 3000;
+    }
+
+    if (this.smoothedRttMs <= 80) {
+      return 5000;
+    }
+
+    if (this.smoothedRttMs <= 180) {
+      return 3000;
+    }
+
+    return 2000;
+  }
+
+  /** 内部状態を初期化する */
+  public reset(): void {
+    this.smoothedOffsetMs = null;
+    this.smoothedRttMs = null;
+  }
+
+  private smoothValue(
+    current: number | null,
+    measured: number,
+    alpha: number,
+  ): number {
+    if (current === null) {
+      return measured;
+    }
+
+    return current * (1 - alpha) + measured * alpha;
+  }
+}