Two-Factor Authentication is an additional security measure that is enabled by default for all accounts using your registered email address (login code sent for each login) as the second factor besides your password.
If you are looking to use your mobile phone instead of email for two-factor authentication, we provide the following two alternatives:
1. SMS (Text)
2. Authentication App (Google Authenticator, Microsoft Authenticator)
*bitFlyer recommends using an Authentication Application.
For details on how to set up Two-Factor Authentication using mobile, please see How do I set up or modify Two-Factor Authentication?