diff --git a/apps/client/src/scenes/game/application/GameNetworkSync.ts b/apps/client/src/scenes/game/application/GameNetworkSync.ts
index 6e7422a..0bf4f13 100644
--- a/apps/client/src/scenes/game/application/GameNetworkSync.ts
+++ b/apps/client/src/scenes/game/application/GameNetworkSync.ts
@@ -49,8 +49,9 @@
     }
   };
 
-  private handleUpdatePlayers = (serverPlayers: playerTypes.PlayerData[]) => {
-    serverPlayers.forEach((playerData) => {
+  private handleUpdatePlayers = (changedPlayers: playerTypes.PlayerData[]) => {
+    // UPDATE_PLAYERS は差分のみ届くため，対象IDだけ上書き更新する
+    changedPlayers.forEach((playerData) => {
       if (playerData.id === this.myId) return;
 
       const target = this.players[playerData.id];
diff --git a/apps/server/src/domains/game/application/services/GamePlayerOperationService.ts b/apps/server/src/domains/game/application/services/GamePlayerOperationService.ts
index 1a3514e..e00cf54 100644
--- a/apps/server/src/domains/game/application/services/GamePlayerOperationService.ts
+++ b/apps/server/src/domains/game/application/services/GamePlayerOperationService.ts
@@ -20,8 +20,8 @@
   public movePlayer(id: string, x: number, y: number): void {
     const roomId = this.playerToRoom.get(id);
     if (!roomId) {
-      logEvent("GameSessionService", {
-        event: "MOVE",
+      logEvent("GamePlayerOperationService", {
+        event: "PLAYER_MOVE",
         result: "ignored_player_not_in_session",
         socketId: id,
       });
@@ -34,8 +34,8 @@
   public removePlayer(id: string): void {
     const roomId = this.playerToRoom.get(id);
     if (!roomId) {
-      logEvent("GameSessionService", {
-        event: "REMOVE_PLAYER",
+      logEvent("GamePlayerOperationService", {
+        event: "PLAYER_REMOVE",
         result: "ignored_player_not_in_session",
         socketId: id,
       });
@@ -61,8 +61,8 @@
       session.dispose();
       this.sessions.delete(roomId);
       this.roomToPlayers.delete(roomId);
-      logEvent("GameSessionService", {
-        event: "REMOVE_PLAYER",
+      logEvent("GamePlayerOperationService", {
+        event: "PLAYER_REMOVE",
         result: "session_disposed_empty_room",
         roomId,
         socketId: id,
diff --git a/apps/server/src/domains/game/application/services/GameSessionLifecycleService.ts b/apps/server/src/domains/game/application/services/GameSessionLifecycleService.ts
index 7e648fc..52261b6 100644
--- a/apps/server/src/domains/game/application/services/GameSessionLifecycleService.ts
+++ b/apps/server/src/domains/game/application/services/GameSessionLifecycleService.ts
@@ -34,8 +34,8 @@
     onGameEnd: () => void
   ) {
     if (this.sessions.has(roomId)) {
-      logEvent("GameSessionService", {
-        event: "START_GAME_LOOP",
+      logEvent("GameSessionLifecycleService", {
+        event: "SESSION_START",
         result: "ignored_already_running",
         roomId,
       });
@@ -58,8 +58,8 @@
       onGameEnd();
     });
 
-    logEvent("GameSessionService", {
-      event: "START_GAME_LOOP",
+    logEvent("GameSessionLifecycleService", {
+      event: "SESSION_START",
       result: "started",
       roomId,
       playerCount: playerIds.length,
diff --git a/apps/server/src/domains/game/application/useCases/startGameUseCase.ts b/apps/server/src/domains/game/application/useCases/startGameUseCase.ts
index 9f95b56..c3a31d2 100644
--- a/apps/server/src/domains/game/application/useCases/startGameUseCase.ts
+++ b/apps/server/src/domains/game/application/useCases/startGameUseCase.ts
@@ -31,7 +31,9 @@
     roomId,
     playerIds,
     (tickData) => {
-      output.publishUpdatePlayersToRoom(roomId, tickData.players);
+      if (tickData.players.length > 0) {
+        output.publishUpdatePlayersToRoom(roomId, tickData.players);
+      }
 
       if (tickData.cellUpdates.length > 0) {
         output.publishMapCellUpdatesToRoom(roomId, tickData.cellUpdates);
diff --git a/apps/server/src/domains/game/loop/GameLoop.ts b/apps/server/src/domains/game/loop/GameLoop.ts
index 889a66a..18aa1d5 100644
--- a/apps/server/src/domains/game/loop/GameLoop.ts
+++ b/apps/server/src/domains/game/loop/GameLoop.ts
@@ -29,6 +29,7 @@
   private endMonotonicTimeMs: number = 0;
   private nextTickAtMs: number = 0;
   private readonly maxCatchUpTicks: number = 3;
+  private lastSentPlayers: Map<string, TickData["players"][number]> = new Map();
 
   constructor(
     private roomId: string,
@@ -47,6 +48,7 @@
     this.startMonotonicTimeMs = nowMs;
     this.endMonotonicTimeMs = nowMs + config.GAME_CONFIG.GAME_DURATION_SEC * 1000;
     this.nextTickAtMs = nowMs + this.tickRate;
+    this.lastSentPlayers.clear();
     this.isRunning = true;
     this.scheduleNextTick();
 
@@ -101,7 +103,7 @@
   }
 
   private processSingleTick(): void {
-    const playersData: TickData["players"] = [];
+    const changedPlayers: TickData["players"] = [];
 
     // 1. 各プレイヤーの座標処理とマス塗りの判定
     this.players.forEach((player) => {
@@ -112,12 +114,31 @@
       }
 
       // 送信用のプレイヤーデータを構築
-      playersData.push({
+      const playerData = {
         id: player.id,
         x: player.x,
         y: player.y,
         teamId: player.teamId,
-      });
+      };
+
+      const lastSentPlayer = this.lastSentPlayers.get(player.id);
+      const isChanged =
+        !lastSentPlayer ||
+        lastSentPlayer.x !== playerData.x ||
+        lastSentPlayer.y !== playerData.y ||
+        lastSentPlayer.teamId !== playerData.teamId;
+
+      if (isChanged) {
+        changedPlayers.push(playerData);
+        this.lastSentPlayers.set(player.id, playerData);
+      }
+    });
+
+    // ルームから離脱したプレイヤーの送信状態をクリーンアップする
+    Array.from(this.lastSentPlayers.keys()).forEach((playerId) => {
+      if (!this.players.has(playerId)) {
+        this.lastSentPlayers.delete(playerId);
+      }
     });
 
     // 2. マスの差分（Diff）を取得
@@ -125,7 +146,7 @@
 
     // 3. 通信層（GameHandler）へデータを渡す
     this.onTick({
-      players: playersData,
+      players: changedPlayers,
       cellUpdates: cellUpdates,
     });
   }
@@ -134,6 +155,7 @@
     if (!this.isRunning) return;
 
     this.isRunning = false;
+    this.lastSentPlayers.clear();
 
     if (this.loopId) {
       clearTimeout(this.loopId);