diff --git a/apps/server/src/domains/game/GameManager.ts b/apps/server/src/domains/game/GameManager.ts
index c6f5a6f..c94a91b 100644
--- a/apps/server/src/domains/game/GameManager.ts
+++ b/apps/server/src/domains/game/GameManager.ts
@@ -13,14 +13,16 @@
 export class GameManager {
   private sessions: Map<string, GameRoomSession>;
   private playerToRoom: Map<string, string>;
+  private roomToPlayers: Map<string, Set<string>>;
   private lifecycleService: GameSessionLifecycleService;
   private playerOperationService: GamePlayerOperationService;
 
   constructor() {
     this.sessions = new Map();
     this.playerToRoom = new Map();
-    this.lifecycleService = new GameSessionLifecycleService(this.sessions, this.playerToRoom);
-    this.playerOperationService = new GamePlayerOperationService(this.sessions, this.playerToRoom);
+    this.roomToPlayers = new Map();
+    this.lifecycleService = new GameSessionLifecycleService(this.sessions, this.playerToRoom, this.roomToPlayers);
+    this.playerOperationService = new GamePlayerOperationService(this.sessions, this.playerToRoom, this.roomToPlayers);
   }
 
   // 外部（GameHandlerなど）から開始時刻を取得できるようにする
diff --git a/apps/server/src/domains/game/application/services/GamePlayerOperationService.ts b/apps/server/src/domains/game/application/services/GamePlayerOperationService.ts
index e6af25b..1a3514e 100644
--- a/apps/server/src/domains/game/application/services/GamePlayerOperationService.ts
+++ b/apps/server/src/domains/game/application/services/GamePlayerOperationService.ts
@@ -7,12 +7,14 @@
 
 type SessionStore = Map<string, GameRoomSession>;
 type PlayerRoomIndex = Map<string, string>;
+type RoomPlayersIndex = Map<string, Set<string>>;
 
 /** プレイヤー移動とセッション離脱処理を提供するサービス */
 export class GamePlayerOperationService {
   constructor(
     private sessions: SessionStore,
-    private playerToRoom: PlayerRoomIndex
+    private playerToRoom: PlayerRoomIndex,
+    private roomToPlayers: RoomPlayersIndex
   ) {}
 
   public movePlayer(id: string, x: number, y: number): void {
@@ -43,15 +45,22 @@
     const session = this.sessions.get(roomId);
     if (!session) {
       this.playerToRoom.delete(id);
+      this.roomToPlayers.get(roomId)?.delete(id);
+
+      if (this.roomToPlayers.get(roomId)?.size === 0) {
+        this.roomToPlayers.delete(roomId);
+      }
       return;
     }
 
     const removed = session.removePlayer(id);
     this.playerToRoom.delete(id);
+    this.roomToPlayers.get(roomId)?.delete(id);
 
     if (removed && session.isEmpty()) {
       session.dispose();
       this.sessions.delete(roomId);
+      this.roomToPlayers.delete(roomId);
       logEvent("GameSessionService", {
         event: "REMOVE_PLAYER",
         result: "session_disposed_empty_room",
diff --git a/apps/server/src/domains/game/application/services/GameSessionLifecycleService.ts b/apps/server/src/domains/game/application/services/GameSessionLifecycleService.ts
index 0b22870..7e648fc 100644
--- a/apps/server/src/domains/game/application/services/GameSessionLifecycleService.ts
+++ b/apps/server/src/domains/game/application/services/GameSessionLifecycleService.ts
@@ -9,12 +9,14 @@
 
 type SessionStore = Map<string, GameRoomSession>;
 type PlayerRoomIndex = Map<string, string>;
+type RoomPlayersIndex = Map<string, Set<string>>;
 
 /** ゲームセッションのライフサイクル操作を提供するサービス */
 export class GameSessionLifecycleService {
   constructor(
     private sessions: SessionStore,
-    private playerToRoom: PlayerRoomIndex
+    private playerToRoom: PlayerRoomIndex,
+    private roomToPlayers: RoomPlayersIndex
   ) {}
 
   public getRoomStartTime(roomId: string): number | undefined {
@@ -42,10 +44,12 @@
 
     const tickRate = config.GAME_CONFIG.PLAYER_POSITION_UPDATE_MS;
     const session = new GameRoomSession(roomId, playerIds);
+    const roomPlayerSet = new Set(playerIds);
 
     playerIds.forEach((playerId) => {
       this.playerToRoom.set(playerId, roomId);
     });
+    this.roomToPlayers.set(roomId, roomPlayerSet);
 
     this.sessions.set(roomId, session);
     session.start(tickRate, onTick, () => {
@@ -63,10 +67,14 @@
   }
 
   private clearRoomPlayerIndex(roomId: string): void {
-    Array.from(this.playerToRoom.entries()).forEach(([playerId, mappedRoomId]) => {
-      if (mappedRoomId === roomId) {
-        this.playerToRoom.delete(playerId);
-      }
+    const roomPlayerSet = this.roomToPlayers.get(roomId);
+    if (!roomPlayerSet) {
+      return;
+    }
+
+    roomPlayerSet.forEach((playerId) => {
+      this.playerToRoom.delete(playerId);
     });
+    this.roomToPlayers.delete(roomId);
   }
 }
\ No newline at end of file