This explains on how to set up two-step authentication by using a "secret key" in Google Authenticator (with Google Authenticator system or an authentication application).
How to set up (example images are for iOS)
1. If you are setting up for the first time, please launch an authentication application and tap "開始".
※For Android, please tap "使ってみる".
2. Please tap "既存のアカウントをインポートしますか？".
3. Please launch the bitbank application, tap "settings" from the menu, and then tap "設定" for two-step authentication.
4. Please tap "設定を始める".
5. Please tap the down arrow beside "シークレットキーを使う".
6. "Secret Key" will appear then tap the "Key (a character string)" to copy it.
7. Go back to Google Authenticator and put a name in the "Account" field as you can relate to the key. Please paste the copied "Key (a character string)" into the "キー" field and then tap "追加".
8. An account will be created after you tap "追加" and a 6-digit "Authentication Code" will be displayed.
9. Go back to the bitbank application, please paste a 6-digit "authentication code" displayed in Google Authenticator and then tap "設定する".
10. That's all you have to do to set up two-step authentication by using a secret key.