diff --git a/backend/app/Http/Controllers/Api/FathersController.php b/backend/app/Http/Controllers/Api/FathersController.php
index 6f7cc33c..b16e7e54 100644
--- a/backend/app/Http/Controllers/Api/FathersController.php
+++ b/backend/app/Http/Controllers/Api/FathersController.php
@@ -424,6 +424,13 @@ class FathersController extends Controller {
             return ['status_code' => 422, 'error_messages' => $validate->errors()];
         }
 
+        if (request()->route()->action['as'] == 'fua') {
+            $rel = FatherRelation::select('id')->where('father_id', (int)$father_id)->count();
+            if ($rel > (int)$r->relation_limit) {
+                return ['status_code' => 401, 'error_messages' => '既に設定しているリレーション以下には変更できません。'];
+            }
+        }
+
         $update = [
             'email' => $r->email,
             'company' => $r->company,