diff --git a/app/Http/Controllers/MemberController.php b/app/Http/Controllers/MemberController.php
index 8a1f3bf..046c5e3 100644
--- a/app/Http/Controllers/MemberController.php
+++ b/app/Http/Controllers/MemberController.php
@@ -36,7 +36,7 @@ public function store(Request $request)
         if ($group == null) abort(404, __('errors.invalid_invitation'));
 
         $validator = Validator::make($request->all(), [
-            'nickname' => ['required', 'string', 'min:1', 'max:15', new UniqueNickname($group->id)],
+            'nickname' => ['required', 'string', 'min:1', 'max:15', new UniqueNickname($group->id, $request->merge_with_member_id)],
             'merge_with_member_id' => ['nullable', new IsMember($group)]
         ]);
         if ($validator->fails()) abort(400, $validator->errors()->first());
diff --git a/app/Rules/UniqueNickname.php b/app/Rules/UniqueNickname.php
index 9457c57..40ef063 100644
--- a/app/Rules/UniqueNickname.php
+++ b/app/Rules/UniqueNickname.php
@@ -9,12 +9,14 @@ class UniqueNickname implements Rule
 {
     /**
      * Create a new rule instance.
-     * @param mixed $group_id
+     * @param int $group_id
+     * @param int|null $except_member_id
      * @return void
      */
-    public function __construct($group_id)
+    public function __construct($group_id, $except_member_id)
     {
         $this->group = Group::find($group_id);
+        $this->except_member_id = $except_member_id;
     }
 
     /**
@@ -26,7 +28,10 @@ public function __construct($group_id)
      */
     public function passes($attribute, $value)
     {
-        return $this->group->members->where('member_data.nickname', $value)->count() == 0;
+        return $this->group->members
+                ->where('member_data.nickname', $value)
+                ->except([$this->except_member_id])
+                ->count() == 0;
     }
 
     /**