In case you have a previous phone with two-step authentication, you can deactivate its setting by yourself and reconfigure it with a new phone.
Please follow the steps below to deactivate.
1．Launch the authentication app you downloaded such as Google Authenticator or IIJ Smartkey etc.
2． Select the "セキュリティ（Security）" menu on our website and press the "解除（Deactivate）" button for "二段階認証（two-step authentication）".
3．Enter the 6-digit code displayed in the authentication application app on the "Two-Step Authentication Settings" box in the deactivate page.
4．If you have set up SMS authentication, you will be asked to enter the SMS authentication code. Press the "SMS確認コードを取得（Get SMS authentication code）" button and enter the 6-digit code sent to you in the "SMS*" box.
5．Press the "解除リクエスト（Deactivation Request）" button.
As for steps 3 and 4, the authentication code has an expiration date of about 30 seconds, so please process it quickly.