JP domains do not have any auth codes or domain passwords for transfer. However, some registrars ask their customers to provide REG ID (number) of the domains in order to initiate transfer.
If the gaining registrar demands the domain's REG ID, please follow the procedures below to obtain the domain name's REG ID.
1) Log into your account and go to the "Domain List" page.
2) Click the domain name and go to the "Domain Information" page.
3) Scroll down a little bit, then you will see the domain name's "REG-ID" in the "Registrant Information".