app/Http/Controllers/RegularContractCreateController.php

@@ -825,7 +825,8 @@ class RegularContractCreateController extends Controller
         ]);
 
         if ($validator->fails()) {
-            return back()
+            // ここでリダイレクト先を明示的に指定
+            return redirect()->route('regular_contract.create_confirm_next', ['contract_id' => $contract_id])
                 ->withErrors($validator)
                 ->withInput();
         }

routes/web.php

@@ -125,7 +125,7 @@ Route::get('/regular-contract/upload_identity_success', [RegularContractCreateCo
 Route::get('/regular-contract/upload_identity_fail', [RegularContractCreateController::class, 'showUploadIdentityFail'])->name('regular_contract.upload_identity_fail');
 Route::get('regular_contract/create_confirm', [RegularContractCreateController::class, 'createConfirm'])->name('regular_contract.create_confirm');
 Route::post('/regular_contract/create_confirm_next/{contract_id}', [RegularContractCreateController::class, 'createConfirmNext'])->name('regular_contract.create_confirm_next');
-Route::post('/regular_contract/create_select_period', [RegularContractCreateController::class, 'selectPeriod'])->name('regular_contract.create_select_period');
+Route::match(['get', 'post'], 'regular_contract/create_select_period', [RegularContractCreateController::class, 'selectPeriod'])->name('regular_contract.create_select_period');
 
 // 定期契約更新
 Route::get('regular_contract/update', [RegularContractController::class, 'showInfo'])->name('regular_contract.update');