diff --git a/src/Application/Gaas.GobbletGobblers.Application/UseCases/CreateGameRequest.cs b/src/Application/Gaas.GobbletGobblers.Application/UseCases/CreateGameRequest.cs
index 4306551..8528022 100644
--- a/src/Application/Gaas.GobbletGobblers.Application/UseCases/CreateGameRequest.cs
+++ b/src/Application/Gaas.GobbletGobblers.Application/UseCases/CreateGameRequest.cs
@@ -2,7 +2,7 @@
 {
     public class CreateGameRequest
     {
-        public Guid PlayerId { get; set; }
+        public Guid? PlayerId { get; set; }
 
         public string PlayerName { get; set; }
     }
diff --git a/src/Application/Gaas.GobbletGobblers.Application/UseCases/CreateGameUseCase.cs b/src/Application/Gaas.GobbletGobblers.Application/UseCases/CreateGameUseCase.cs
index 18fc67c..c24c444 100644
--- a/src/Application/Gaas.GobbletGobblers.Application/UseCases/CreateGameUseCase.cs
+++ b/src/Application/Gaas.GobbletGobblers.Application/UseCases/CreateGameUseCase.cs
@@ -16,7 +16,7 @@ public async Task<GameModel> ExecuteAsync(CreateGameRequest request, IRepository
                 throw new Exception();
 
             // 改
-            var player = new Player(request.PlayerId);
+            var player = request.PlayerId == null ? new Player() : new Player(request.PlayerId.Value);
             player.Nameself(request.PlayerName);
 
             game = new Game();
diff --git a/src/Application/Gaas.GobbletGobblers.Application/UseCases/JoinGameRequest.cs b/src/Application/Gaas.GobbletGobblers.Application/UseCases/JoinGameRequest.cs
index 7dd10b6..b05215a 100644
--- a/src/Application/Gaas.GobbletGobblers.Application/UseCases/JoinGameRequest.cs
+++ b/src/Application/Gaas.GobbletGobblers.Application/UseCases/JoinGameRequest.cs
@@ -4,7 +4,7 @@ public class JoinGameRequest
     {
         public Guid Id { get; set; }
 
-        public Guid PlayerId { get; set; }
+        public Guid? PlayerId { get; set; }
 
         public string PlayerName { get; set; }
     }
diff --git a/src/Application/Gaas.GobbletGobblers.Application/UseCases/JoinGameUseCase.cs b/src/Application/Gaas.GobbletGobblers.Application/UseCases/JoinGameUseCase.cs
index 75aab19..dbf1cfb 100644
--- a/src/Application/Gaas.GobbletGobblers.Application/UseCases/JoinGameUseCase.cs
+++ b/src/Application/Gaas.GobbletGobblers.Application/UseCases/JoinGameUseCase.cs
@@ -14,7 +14,7 @@ public async Task<GameModel> ExecuteAsync(JoinGameRequest request, IRepository r
                 throw new Exception();
 
             // 改
-            var player = new Player(request.PlayerId);
+            var player = request.PlayerId == null ? new Player() : new Player(request.PlayerId.Value);
             player.Nameself(request.PlayerName);
             game.JoinPlayer(player);