diff --git a/src/PaymentMethod/SoftbankPayload.php b/src/PaymentMethod/SoftbankPayload.php
index d1fe7db..8c80dd4 100644
--- a/src/PaymentMethod/SoftbankPayload.php
+++ b/src/PaymentMethod/SoftbankPayload.php
@@ -203,7 +203,13 @@ public function setHashKey($hashKey): Payload
      */ 
     public function getAuthUsername(): ?string
     {
-        return $this->merchantId . $this->serviceId;
+        $username = $this->merchantId . $this->serviceId;
+
+        if ($username) {
+            return $username;
+        }
+
+        return config('laravel-softbank.merchant_id') . config('laravel-softbank.service_id');
     }
 
     /**
@@ -214,7 +220,13 @@ public function getAuthUsername(): ?string
      */ 
     public function getAuthPassword(): ?string
     {
-        return $this->hashKey;
+        $password = $this->hashKey;
+
+        if ($password) {
+            return $password;
+        }
+
+        return config('laravel-softbank.hash_key');
     }
 
     /**
diff --git a/src/Traits/HttpClient/SoftbankHttpClient.php b/src/Traits/HttpClient/SoftbankHttpClient.php
index be2e1b8..59b345c 100644
--- a/src/Traits/HttpClient/SoftbankHttpClient.php
+++ b/src/Traits/HttpClient/SoftbankHttpClient.php
@@ -11,9 +11,7 @@ trait SoftbankHttpClient
 
     public function postData(string $xmlData, array $auth = []): Response
     {
-        return Http::withBasicAuth(
-            $auth['username'] ?? config('laravel-softbank.merchant_id') . config('laravel-softbank.service_id'), 
-            $auth['password'] ?? config('laravel-softbank.hash_key'))
+        return Http::withBasicAuth($auth['username'], $auth['password'])
             ->withBody($xmlData, 'application/xml')
             ->post(config('laravel-softbank.api_endpoint') ?? $this->apiEndpoint);
     }