diff --git a/apps/server/src/domains/game/application/useCases/executeStartGameUseCase.ts b/apps/server/src/domains/game/application/useCases/executeStartGameUseCase.ts
new file mode 100644
index 0000000..760cb58
--- /dev/null
+++ b/apps/server/src/domains/game/application/useCases/executeStartGameUseCase.ts
@@ -0,0 +1,66 @@
+import { protocol, roomConsts } from "@repo/shared";
+import { GameManager } from "@server/domains/game/GameManager";
+import { RoomManager } from "@server/domains/room/RoomManager";
+
+type EmitToRoom = (roomId: string, event: string, payload?: unknown) => void;
+
+type ExecuteStartGameUseCaseParams = {
+  ownerId: string;
+  gameManager: GameManager;
+  roomManager: RoomManager;
+  emitToRoom: EmitToRoom;
+};
+
+export const executeStartGameUseCase = ({
+  ownerId,
+  gameManager,
+  roomManager,
+  emitToRoom,
+}: ExecuteStartGameUseCaseParams) => {
+  const room = roomManager.getRoomByOwnerId(ownerId);
+  if (!room) {
+    console.log("[GameHandler] START_GAME ignored (no room)", { socketId: ownerId });
+    return;
+  }
+
+  if (room.status === roomConsts.RoomPhase.PLAYING) {
+    console.log("[GameHandler] START_GAME ignored (already playing)", { roomId: room.roomId });
+    return;
+  }
+
+  console.log("[GameHandler] START_GAME accepted", {
+    roomId: room.roomId,
+    ownerId,
+    totalPlayers: room.players.length,
+  });
+
+  room.status = roomConsts.RoomPhase.PLAYING;
+
+  const playerIds = room.players.map((p: { id: string }) => p.id);
+
+  room.players.forEach((p: { id: string }) => {
+    gameManager.addPlayer(p.id);
+  });
+
+  gameManager.startGameLoop(
+    room.roomId,
+    playerIds,
+    (tickData) => {
+      tickData.players.forEach((playerData) => {
+        emitToRoom(room.roomId, protocol.SocketEvents.UPDATE_PLAYER, playerData);
+      });
+
+      if (tickData.cellUpdates.length > 0) {
+        emitToRoom(room.roomId, protocol.SocketEvents.UPDATE_MAP_CELLS, tickData.cellUpdates);
+      }
+    },
+    () => {
+      console.log(`[GameHandler] ルーム ${room.roomId} のゲームが終了しました (3分経過)`);
+      emitToRoom(room.roomId, protocol.SocketEvents.GAME_END);
+      room.status = roomConsts.RoomPhase.WAITING;
+    }
+  );
+
+  const startTime = gameManager.getRoomStartTime(room.roomId) || Date.now();
+  emitToRoom(room.roomId, protocol.SocketEvents.GAME_START, { startTime });
+};
diff --git a/apps/server/src/domains/game/handlers/onStartGame.ts b/apps/server/src/domains/game/handlers/onStartGame.ts
index e27d612..3b759f3 100644
--- a/apps/server/src/domains/game/handlers/onStartGame.ts
+++ b/apps/server/src/domains/game/handlers/onStartGame.ts
@@ -1,7 +1,7 @@
 import { Server } from "socket.io";
 import { GameManager } from "@server/domains/game/GameManager";
 import { RoomManager } from "@server/domains/room/RoomManager";
-import { protocol, roomConsts } from "@repo/shared";
+import { executeStartGameUseCase } from "@server/domains/game/application/useCases/executeStartGameUseCase";
 
 export const onStartGame = (
   io: Server,
@@ -9,50 +9,17 @@
   roomManager: RoomManager,
   ownerId: string
 ) => {
-  const room = roomManager.getRoomByOwnerId(ownerId);
-  if (!room) {
-    console.log("[GameHandler] START_GAME ignored (no room)", { socketId: ownerId });
-    return;
-  }
-
-  if (room.status === roomConsts.RoomPhase.PLAYING) {
-    console.log("[GameHandler] START_GAME ignored (already playing)", { roomId: room.roomId });
-    return;
-  }
-
-  console.log("[GameHandler] START_GAME accepted", {
-    roomId: room.roomId,
+  executeStartGameUseCase({
     ownerId,
-    totalPlayers: room.players.length,
-  });
-
-  room.status = roomConsts.RoomPhase.PLAYING;
-
-  const playerIds = room.players.map((p: { id: string }) => p.id);
-
-  room.players.forEach((p: { id: string }) => {
-    gameManager.addPlayer(p.id);
-  });
-
-  gameManager.startGameLoop(
-    room.roomId,
-    playerIds,
-    (tickData) => {
-      tickData.players.forEach((playerData) => {
-        io.to(room.roomId).emit(protocol.SocketEvents.UPDATE_PLAYER, playerData);
-      });
-
-      if (tickData.cellUpdates.length > 0) {
-        io.to(room.roomId).emit(protocol.SocketEvents.UPDATE_MAP_CELLS, tickData.cellUpdates);
+    gameManager,
+    roomManager,
+    emitToRoom: (roomId, event, payload) => {
+      if (payload === undefined) {
+        io.to(roomId).emit(event);
+        return;
       }
-    },
-    () => {
-      console.log(`[GameHandler] ルーム ${room.roomId} のゲームが終了しました (3分経過)`);
-      io.to(room.roomId).emit(protocol.SocketEvents.GAME_END);
-      room.status = roomConsts.RoomPhase.WAITING;
-    }
-  );
 
-  const startTime = gameManager.getRoomStartTime(room.roomId) || Date.now();
-  io.to(room.roomId).emit(protocol.SocketEvents.GAME_START, { startTime });
+      io.to(roomId).emit(event, payload);
+    },
+  });
 };