diff --git a/ClientTester/EchoClientTester/Packet/PacketBody.cs b/ClientTester/EchoClientTester/Packet/PacketBody.cs
index ecae837..638810a 100644
--- a/ClientTester/EchoClientTester/Packet/PacketBody.cs
+++ b/ClientTester/EchoClientTester/Packet/PacketBody.cs
@@ -516,6 +516,31 @@ public class DamagePacket
     }
 }
 
+// ============================================================
+// 에러
+// ============================================================
+
+public enum ErrorCode : int
+{
+    // 파티 (10021~)
+    PARTY_ALREADY_IN_PARTY = 10021,
+    PARTY_JOIN_FAILED      = 10022,
+    PARTY_NOT_IN_PARTY     = 10023,
+    PARTY_DELETE_FAILED    = 10024,
+}
+
+// ERROR (서버 -> 클라)
+[ProtoContract]
+public class ErrorPacket
+{
+    [ProtoMember(1)]
+    public ErrorCode Code
+    {
+        get;
+        set;
+    }
+}
+
 // ============================================================
 // 파티
 // ============================================================
diff --git a/ClientTester/EchoClientTester/Packet/PacketHeader.cs b/ClientTester/EchoClientTester/Packet/PacketHeader.cs
index 886af11..5210021 100644
--- a/ClientTester/EchoClientTester/Packet/PacketHeader.cs
+++ b/ClientTester/EchoClientTester/Packet/PacketHeader.cs
@@ -52,7 +52,10 @@ public enum PacketCode : ushort
     UPDATE_PARTY,
 
     // 파티 참가/탈퇴/생성/해산 요청 (클라 -> 서버)
-    REQUEST_PARTY
+    REQUEST_PARTY,
+
+    // 요청 실패 응답 (서버 -> 클라)
+    ERROR = 9999
 }
 
 public class PacketHeader
diff --git a/MMOTestServer/MMOserver/Game/GameServer.cs b/MMOTestServer/MMOserver/Game/GameServer.cs
index 80e95be..0c00419 100644
--- a/MMOTestServer/MMOserver/Game/GameServer.cs
+++ b/MMOTestServer/MMOserver/Game/GameServer.cs
@@ -484,6 +484,7 @@ public class GameServer : ServerBase
             {
                 if (!pm.CreateParty(hashKey, req.PartyName, out PartyInfo? party))
                 {
+                    SendError(peer, ErrorCode.PARTY_ALREADY_IN_PARTY);
                     return;
                 }
 
@@ -503,6 +504,7 @@ public class GameServer : ServerBase
             {
                 if (!pm.JoinParty(hashKey, req.PartyId, out PartyInfo? party))
                 {
+                    SendError(peer, ErrorCode.PARTY_JOIN_FAILED);
                     return;
                 }
 
@@ -521,6 +523,7 @@ public class GameServer : ServerBase
             {
                 if (!pm.LeaveParty(hashKey, out PartyInfo? party))
                 {
+                    SendError(peer, ErrorCode.PARTY_NOT_IN_PARTY);
                     return;
                 }
 
@@ -544,6 +547,7 @@ public class GameServer : ServerBase
             {
                 if (!pm.DeleteParty(hashKey, req.PartyId, out PartyInfo? party))
                 {
+                    SendError(peer, ErrorCode.PARTY_DELETE_FAILED);
                     return;
                 }
 
@@ -560,6 +564,13 @@ public class GameServer : ServerBase
         }
     }
 
+    private void SendError(NetPeer peer, ErrorCode code)
+    {
+        ErrorPacket err = new ErrorPacket { Code = code };
+        byte[] data = PacketSerializer.Serialize<ErrorPacket>((ushort)PacketCode.ERROR, err);
+        SendTo(peer, data);
+    }
+
     private void BroadcastToUsers(IEnumerable<int> userIds, byte[] data, DeliveryMethod method = DeliveryMethod.ReliableOrdered)
     {
         foreach (int userId in userIds)
diff --git a/MMOTestServer/MMOserver/Packet/PacketBody.cs b/MMOTestServer/MMOserver/Packet/PacketBody.cs
index ee3a61d..fe1a96c 100644
--- a/MMOTestServer/MMOserver/Packet/PacketBody.cs
+++ b/MMOTestServer/MMOserver/Packet/PacketBody.cs
@@ -516,6 +516,31 @@ public class DamagePacket
     }
 }
 
+// ============================================================
+// 에러
+// ============================================================
+
+public enum ErrorCode : int
+{
+    // 파티 (10021~)
+    PARTY_ALREADY_IN_PARTY = 10021,
+    PARTY_JOIN_FAILED      = 10022,
+    PARTY_NOT_IN_PARTY     = 10023,
+    PARTY_DELETE_FAILED    = 10024,
+}
+
+// ERROR (서버 -> 클라)
+[ProtoContract]
+public class ErrorPacket
+{
+    [ProtoMember(1)]
+    public ErrorCode Code
+    {
+        get;
+        set;
+    }
+}
+
 // ============================================================
 // 파티
 // ============================================================
diff --git a/MMOTestServer/MMOserver/Packet/PacketHeader.cs b/MMOTestServer/MMOserver/Packet/PacketHeader.cs
index 8f24111..aca58a9 100644
--- a/MMOTestServer/MMOserver/Packet/PacketHeader.cs
+++ b/MMOTestServer/MMOserver/Packet/PacketHeader.cs
@@ -52,7 +52,10 @@ public enum PacketCode : ushort
     UPDATE_PARTY,
 
     // 파티 참가/탈퇴/생성/해산 요청 (클라 -> 서버)
-    REQUEST_PARTY
+    REQUEST_PARTY,
+
+    // 요청 실패 응답 (서버 -> 클라)
+    ERROR = 9999
 }
 
 public class PacketHeader