diff --git a/backend/app/Http/Controllers/Api/ChildrenController.php b/backend/app/Http/Controllers/Api/ChildrenController.php
index 65f66ffb..dd34554d 100644
--- a/backend/app/Http/Controllers/Api/ChildrenController.php
+++ b/backend/app/Http/Controllers/Api/ChildrenController.php
@@ -29,9 +29,20 @@ class ChildrenController extends Controller {
     }
 
     public function registerTemporary (Request $r) {
+        // 電話番号の文字数。
+        Validator::extend('tel_size', function ($attribute, $value, $params, $validator) {
+            try {
+                return strlen((string)$value) == 10 || strlen((string)$value) == 11;
+            } catch (\Throwable $e) {
+                Log::critical($e->getMessage());
+                return false;
+            }
+        });
+
         $validate = Validator::make($r->all(), [
-            'tel' => 'required|unique:children|numeric|digits_between:0,99999999999|starts_with:0'
+            'tel' => 'required|unique:children|numeric|starts_with:0|tel_size'
         ]);
+
         if ($validate->fails()) {
             // バリデーションエラー
             return ['status_code' => 422, 'error_messages' => $validate->errors()];
@@ -155,8 +166,18 @@ KIKI承知システムを使って「聞いてない！」「言ってない！
     }
 
     public function requestPassword (Request $r) {
+        // 電話番号の文字数。
+        Validator::extend('tel_size', function ($attribute, $value, $params, $validator) {
+            try {
+                return strlen((string)$value) == 10 || strlen((string)$value) == 11;
+            } catch (\Throwable $e) {
+                Log::critical($e->getMessage());
+                return false;
+            }
+        });
+
         $validate = Validator::make($r->all(), [
-            'tel' => 'required|numeric|digits_between:0,99999999999|starts_with:0',
+            'tel' => 'required|numeric|starts_with:0|tel_size'
         ]);
 
         if ($validate->fails()) {
@@ -395,10 +416,20 @@ https://kikikan.jp/c-account/forgot-password/reset/'.$token;
             return ['status_code' => 400, 'error_messages' => ['子の更新に失敗しました。']];
         }
 
+        // 電話番号の文字数。
+        Validator::extend('tel_size', function ($attribute, $value, $params, $validator) {
+            try {
+                return strlen((string)$value) == 10 || strlen((string)$value) == 11;
+            } catch (\Throwable $e) {
+                Log::critical($e->getMessage());
+                return false;
+            }
+        });
+
         // バリデーションエラー
         $validate = Validator::make($r->all(), [
             'email' => 'required|max:255|email',
-            'tel' => 'required|numeric|digits_between:0,99999999999|starts_with:0',
+            'tel' => 'required|numeric|starts_with:0|tel_size',
             'last_name' => 'required|max:100',
             'first_name' => 'required|max:100',
             'identity' => 'required|max:20|alpha_num',
diff --git a/backend/app/Http/Controllers/Api/FathersController.php b/backend/app/Http/Controllers/Api/FathersController.php
index 1ba34c52..099e3bec 100644
--- a/backend/app/Http/Controllers/Api/FathersController.php
+++ b/backend/app/Http/Controllers/Api/FathersController.php
@@ -113,14 +113,25 @@ class FathersController extends Controller {
     }
 
     public function registerMain (Request $r) {
+        // 電話番号の文字数。
+        Validator::extend('tel_size', function ($attribute, $value, $params, $validator) {
+            try {
+                return strlen((string)$value) == 10 || strlen((string)$value) == 11;
+            } catch (\Throwable $e) {
+                Log::critical($e->getMessage());
+                return false;
+            }
+        });
+
         $validate = Validator::make($r->all(), [
             'token' => 'required',
             'password' => 'required|min:8|max:72|confirmed',
             'company' => 'max:100',
             'image' => 'max:1024|mimes:jpg,png,gif',
             'profile' => 'max:1000',
-            'tel' => 'required|unique:children|numeric|digits_between:0,99999999999|starts_with:0',
+            'tel' => 'required|unique:fathers|numeric|starts_with:0|tel_size',
         ]);
+
         if ($validate->fails()) {
             // バリデーションエラー
             return ['status_code' => 422, 'error_messages' => $validate->errors()];
@@ -310,12 +321,22 @@ class FathersController extends Controller {
             return ['status_code' => 400, 'error_messages' => ['親の更新に失敗しました。']];
         }
 
+        // 電話番号の文字数。
+        Validator::extend('tel_size', function ($attribute, $value, $params, $validator) {
+            try {
+                return strlen((string)$value) == 10 || strlen((string)$value) == 11;
+            } catch (\Throwable $e) {
+                Log::critical($e->getMessage());
+                return false;
+            }
+        });
+
         // バリデーションエラー
         $validate = Validator::make($r->all(), [
             'email' => 'required|max:255|email',
             'company' => 'max:100',
             'profile' => 'max:1000',
-            'tel' => 'required|numeric|digits_between:0,99999999999|starts_with:0',
+            'tel' => 'required|numeric|starts_with:0|tel_size'
         ]);
 
         if ($validate->fails()) {
diff --git a/backend/resources/lang/ja/validation.php b/backend/resources/lang/ja/validation.php
index 527f687f..4993d7e3 100644
--- a/backend/resources/lang/ja/validation.php
+++ b/backend/resources/lang/ja/validation.php
@@ -24,9 +24,9 @@ return [
     'before' => 'The :attribute must be a date before :date.',
     'before_or_equal' => 'The :attribute must be a date before or equal to :date.',
     'between' => [
-        'numeric' => 'The :attribute must be between :min and :max.',
+        'numeric' => ' :attribute は :min ・ :max 文字です。',
         'file' => 'The :attribute must be between :min and :max kilobytes.',
-        'string' => 'The :attribute must be between :min and :max characters.',
+        'string' => ' :attribute は :min ・ :max 文字です。',
         'array' => 'The :attribute must have between :min and :max items.',
     ],
     'boolean' => 'The :attribute field must be true or false.',
@@ -119,6 +119,7 @@ return [
     ],
     'starts_with' => 'The :attribute must start with one of the following: :values.',
     'string' => 'The :attribute must be a string.',
+    'tel_size' => '電話番号は10・11文字です。',
     'timezone' => 'The :attribute must be a valid zone.',
     'unique' => '同じ :attribute は使用できません。',
     'uploaded' => 'The :attribute failed to upload.',