diff --git a/core/group_memory.go b/core/group_memory.go
index 4a08dd8..4c34fb1 100644
--- a/core/group_memory.go
+++ b/core/group_memory.go
@@ -524,14 +524,23 @@ func (gm *groupInMemory) UpdateGroupDocument(gid groupID, frag bson.M) error {
 }
 
 func (gm *groupInMemory) MemberDisconnected(room string, mid primitive.ObjectID) {
-	midstr := mid.Hex()
-	deleted, _ := gm.rh.JSONDel(room, "$._members."+midstr)
+	gid, err := primitive.ObjectIDFromHex(room)
+	if err != nil {
+		return
+	}
+	gd := &groupDoc{
+		id: gid,
+		rh: gm.rh,
+	}
+
+	tid := gd.tid(mid)
+	deleted, _ := gm.rh.JSONDel(room, "$._members."+tid)
 	if deleted > 0 {
 		// 퇴장을 알림
 		gm.sendUpstreamMessage(&wshandler.UpstreamMessage{
 			Target: "#" + room,
 			Body: bson.M{
-				midstr: bson.M{},
+				tid: bson.M{},
 			},
 			Tag: []string{"MemberDocFull"},
 		})