diff --git a/apps/client/src/scenes/game/GameManager.ts b/apps/client/src/scenes/game/GameManager.ts
index 3fd2f58..2b83226 100644
--- a/apps/client/src/scenes/game/GameManager.ts
+++ b/apps/client/src/scenes/game/GameManager.ts
@@ -37,13 +37,21 @@
     this.timer.setGameStart(startTime);
   }
 
+  public getStartCountdownSec(): number {
+    return this.timer.getPreStartRemainingSec();
+  }
+
+  private canAcceptInput(): boolean {
+    return !this.isInputLocked && this.timer.isStarted();
+  }
+
   // 現在の残り秒数を取得する
   public getRemainingTime(): number {
     return this.timer.getRemainingTime();
   }
 
   public placeBomb(): string | null {
-    if (this.isInputLocked) return null;
+    if (!this.canAcceptInput()) return null;
     if (!this.bombManager) return null;
     const placed = this.bombManager.placeBomb();
     if (!placed) return null;
@@ -115,7 +123,10 @@
    * React側からジョイスティックの入力を受け取る
    */
   public setJoystickInput(x: number, y: number) {
-    if (this.isInputLocked) return;
+    if (!this.canAcceptInput()) {
+      this.joystickInput = { x: 0, y: 0 };
+      return;
+    }
     this.joystickInput = { x, y };
   }
 
@@ -194,7 +205,9 @@
   }
 
   /** 爆弾当たり判定の評価結果を受け取り，後続処理へ接続する */
-  private handleBombHitEvaluation(_result: BombHitEvaluationResult | undefined): void {
+  private handleBombHitEvaluation(
+    _result: BombHitEvaluationResult | undefined,
+  ): void {
     // 次フェーズでサーバー通知や被弾演出の接続に利用する
   }
 
diff --git a/apps/client/src/scenes/game/GameScene.tsx b/apps/client/src/scenes/game/GameScene.tsx
index 827b1d1..84663e1 100644
--- a/apps/client/src/scenes/game/GameScene.tsx
+++ b/apps/client/src/scenes/game/GameScene.tsx
@@ -13,14 +13,21 @@
 
 /** メインゲーム画面を描画し入力をゲーム制御へ橋渡しする */
 export function GameScene({ myId }: GameSceneProps) {
-  const { pixiContainerRef, timeLeft, handleInput, handlePlaceBomb } = useGameSceneController(myId);
+  const {
+    pixiContainerRef,
+    timeLeft,
+    startCountdownText,
+    handleInput,
+    handlePlaceBomb,
+  } = useGameSceneController(myId);
 
   return (
     <GameView
       timeLeft={timeLeft}
+      startCountdownText={startCountdownText}
       pixiContainerRef={pixiContainerRef}
       onJoystickInput={handleInput}
       onPlaceBomb={handlePlaceBomb}
     />
   );
-}
\ No newline at end of file
+}
diff --git a/apps/client/src/scenes/game/GameView.tsx b/apps/client/src/scenes/game/GameView.tsx
index 3556e13..904118f 100644
--- a/apps/client/src/scenes/game/GameView.tsx
+++ b/apps/client/src/scenes/game/GameView.tsx
@@ -8,6 +8,7 @@
 /** 表示と入力に必要なプロパティ */
 type Props = {
   timeLeft: string;
+  startCountdownText: string | null;
   pixiContainerRef: React.RefObject<HTMLDivElement>;
   onJoystickInput: (x: number, y: number) => void;
   onPlaceBomb: () => void;
@@ -45,20 +46,51 @@
   zIndex: 1,
 };
 
-const TimerOverlay = ({ timeLeft }: { timeLeft: string }) => <div style={TIMER_STYLE}>{timeLeft}</div>;
+const START_COUNTDOWN_STYLE: React.CSSProperties = {
+  position: "absolute",
+  top: "50%",
+  left: "50%",
+  transform: "translate(-50%, -50%)",
+  zIndex: 30,
+  color: "white",
+  fontSize: "clamp(3rem, 14vw, 8rem)",
+  fontWeight: 900,
+  textShadow: "0 0 16px rgba(0,0,0,0.85)",
+  fontFamily: "monospace",
+  userSelect: "none",
+  WebkitUserSelect: "none",
+  pointerEvents: "none",
+};
+
+const TimerOverlay = ({ timeLeft }: { timeLeft: string }) => (
+  <div style={TIMER_STYLE}>{timeLeft}</div>
+);
 
 /** 画面描画と入力UIをまとめて描画する */
-export const GameView = ({ timeLeft, pixiContainerRef, onJoystickInput, onPlaceBomb }: Props) => {
+export const GameView = ({
+  timeLeft,
+  startCountdownText,
+  pixiContainerRef,
+  onJoystickInput,
+  onPlaceBomb,
+}: Props) => {
   return (
     <div style={ROOT_STYLE}>
       {/* タイマーUIの表示 */}
       <TimerOverlay timeLeft={timeLeft} />
 
+      {startCountdownText && (
+        <div style={START_COUNTDOWN_STYLE}>{startCountdownText}</div>
+      )}
+
       {/* PixiJS Canvas 配置領域 */}
       <div ref={pixiContainerRef} style={PIXI_LAYER_STYLE} />
 
       {/* 入力UI レイヤー */}
-      <GameInputOverlay onJoystickInput={onJoystickInput} onPlaceBomb={onPlaceBomb} />
+      <GameInputOverlay
+        onJoystickInput={onJoystickInput}
+        onPlaceBomb={onPlaceBomb}
+      />
     </div>
   );
 };
diff --git a/apps/client/src/scenes/game/application/GameTimer.ts b/apps/client/src/scenes/game/application/GameTimer.ts
index 7a63c81..b31d726 100644
--- a/apps/client/src/scenes/game/application/GameTimer.ts
+++ b/apps/client/src/scenes/game/application/GameTimer.ts
@@ -21,11 +21,37 @@
     this.gameStartTime = startTime;
   }
 
+  public getStartTime(): number | null {
+    return this.gameStartTime;
+  }
+
+  public isStarted(): boolean {
+    if (!this.gameStartTime) {
+      return false;
+    }
+
+    return this.nowMsProvider() >= this.gameStartTime;
+  }
+
+  public getPreStartRemainingSec(): number {
+    if (!this.gameStartTime) {
+      return 0;
+    }
+
+    const remainingMs = this.gameStartTime - this.nowMsProvider();
+    if (remainingMs <= 0) {
+      return 0;
+    }
+
+    return Math.ceil(remainingMs / 1000);
+  }
+
   public getRemainingTime(): number {
     if (!this.gameStartTime) return config.GAME_CONFIG.GAME_DURATION_SEC;
 
     const elapsedMs = this.nowMsProvider() - this.gameStartTime;
-    const remainingSec = config.GAME_CONFIG.GAME_DURATION_SEC - elapsedMs / 1000;
+    const remainingSec =
+      config.GAME_CONFIG.GAME_DURATION_SEC - elapsedMs / 1000;
 
     return Math.max(0, remainingSec);
   }
diff --git a/apps/client/src/scenes/game/hooks/useGameSceneController.ts b/apps/client/src/scenes/game/hooks/useGameSceneController.ts
index bc060f1..f2ddb49 100644
--- a/apps/client/src/scenes/game/hooks/useGameSceneController.ts
+++ b/apps/client/src/scenes/game/hooks/useGameSceneController.ts
@@ -14,7 +14,8 @@
   return `${mins}:${secs.toString().padStart(2, "0")}`;
 };
 
-const getInitialTimeDisplay = () => formatRemainingTime(config.GAME_CONFIG.GAME_DURATION_SEC);
+const getInitialTimeDisplay = () =>
+  formatRemainingTime(config.GAME_CONFIG.GAME_DURATION_SEC);
 
 /** ゲーム画面の状態と入力ハンドラを提供するフック */
 export const useGameSceneController = (myId: string | null) => {
@@ -22,6 +23,9 @@
   const gameManagerRef = useRef<GameManager | null>(null);
   const inputManagerRef = useRef<GameInputManager | null>(null);
   const [timeLeft, setTimeLeft] = useState(getInitialTimeDisplay());
+  const [startCountdownText, setStartCountdownText] = useState<string | null>(
+    null,
+  );
 
   useEffect(() => {
     if (!pixiContainerRef.current || !myId) return;
@@ -36,12 +40,18 @@
       },
       () => {
         manager.placeBomb();
-      }
+      },
     );
 
     const timerInterval = setInterval(() => {
       const nextDisplay = formatRemainingTime(manager.getRemainingTime());
       setTimeLeft((prev) => (prev === nextDisplay ? prev : nextDisplay));
+
+      const remainingSec = manager.getStartCountdownSec();
+      const nextCountdown = remainingSec > 0 ? String(remainingSec) : null;
+      setStartCountdownText((prev) =>
+        prev === nextCountdown ? prev : nextCountdown,
+      );
     }, config.GAME_CONFIG.TIMER_DISPLAY_UPDATE_MS);
 
     return () => {
@@ -49,6 +59,7 @@
       gameManagerRef.current = null;
       inputManagerRef.current = null;
       clearInterval(timerInterval);
+      setStartCountdownText(null);
     };
   }, [myId]);
 
@@ -63,6 +74,7 @@
   return {
     pixiContainerRef,
     timeLeft,
+    startCountdownText,
     handleInput,
     handlePlaceBomb,
   };
diff --git a/apps/server/src/config/index.ts b/apps/server/src/config/index.ts
index 92a3533..414d45f 100644
--- a/apps/server/src/config/index.ts
+++ b/apps/server/src/config/index.ts
@@ -1,7 +1,13 @@
 import { config as sharedConfig } from "@repo/shared";
 
+const sharedGameConfig =
+  sharedConfig.GAME_CONFIG as typeof sharedConfig.GAME_CONFIG & {
+    GAME_START_DELAY_MS?: number;
+  };
+
 const GAME_CONFIG = {
-  ...sharedConfig.GAME_CONFIG,
+  ...sharedGameConfig,
+  GAME_START_DELAY_MS: sharedGameConfig.GAME_START_DELAY_MS ?? 5000,
   MAX_PLAYERS_PER_ROOM: 100,
 } as const;
 
diff --git a/apps/server/src/domains/game/application/services/GameRoomSession.ts b/apps/server/src/domains/game/application/services/GameRoomSession.ts
index d6a2d72..107f3ab 100644
--- a/apps/server/src/domains/game/application/services/GameRoomSession.ts
+++ b/apps/server/src/domains/game/application/services/GameRoomSession.ts
@@ -9,6 +9,7 @@
   logScopes,
 } from "@server/logging/index";
 import type { gameTypes, GameResultPayload } from "@repo/shared";
+import { config } from "@server/config";
 import { GameLoop } from "../../loop/GameLoop";
 import { Player } from "../../entities/player/Player.js";
 import { MapStore } from "../../entities/map/MapStore";
@@ -29,6 +30,7 @@
   private bombStateStore: BombStateStore;
   private gameLoop: GameLoop | null = null;
   private startTime: number | undefined;
+  private startDelayTimer: NodeJS.Timeout | null = null;
 
   constructor(
     private roomId: string,
@@ -61,7 +63,13 @@
       return;
     }
 
-    this.startTime = Date.now();
+    const gameStartDelayMs = (
+      config.GAME_CONFIG as typeof config.GAME_CONFIG & {
+        GAME_START_DELAY_MS?: number;
+      }
+    ).GAME_START_DELAY_MS;
+    const startDelayMs = Math.max(0, gameStartDelayMs ?? 0);
+    this.startTime = Date.now() + startDelayMs;
     this.gameLoop = new GameLoop(
       this.roomId,
       tickRate,
@@ -78,10 +86,28 @@
       onBotPlaceBomb,
     );
 
-    this.gameLoop.start();
+    if (startDelayMs === 0) {
+      this.gameLoop.start();
+      return;
+    }
+
+    this.startDelayTimer = setTimeout(() => {
+      this.startDelayTimer = null;
+      this.gameLoop?.start();
+    }, startDelayMs);
   }
 
   public movePlayer(id: string, x: number, y: number): void {
+    if (this.startTime && Date.now() < this.startTime) {
+      logEvent(logScopes.GAME_ROOM_SESSION, {
+        event: gameDomainLogEvents.MOVE,
+        result: logResults.IGNORED_INVALID_PAYLOAD,
+        roomId: this.roomId,
+        socketId: id,
+      });
+      return;
+    }
+
     const player = this.players.get(id);
     if (!player) {
       logEvent(logScopes.GAME_ROOM_SESSION, {
@@ -131,6 +157,11 @@
   }
 
   public dispose(): void {
+    if (this.startDelayTimer) {
+      clearTimeout(this.startDelayTimer);
+      this.startDelayTimer = null;
+    }
+
     if (this.gameLoop) {
       this.gameLoop.stop();
       this.gameLoop = null;
diff --git a/packages/shared/src/config/gameConfig.ts b/packages/shared/src/config/gameConfig.ts
index c9ac7ac..8f0ff0f 100644
--- a/packages/shared/src/config/gameConfig.ts
+++ b/packages/shared/src/config/gameConfig.ts
@@ -7,6 +7,7 @@
 export const GAME_CONFIG = {
   // ゲーム進行設定（クライアント/サーバー契約）
   GAME_DURATION_SEC: 30, // 1ゲームの制限時間（3分 = 180秒）
+  GAME_START_DELAY_MS: 5000, // 開始通知から実際にゲーム進行を開始するまでの待機時間（ms）
 
   // ネットワーク同期設定（クライアント/サーバー契約）
   PLAYER_POSITION_UPDATE_MS: 50, // 座標送信間隔（20Hz）