Under the Act on Regulation of the Transmission of Specified Electronic Mail it is illegal to send, or cause to be sent, unsolicited commercial electronic messages. The ACT covers email, text and image-based mobile phone messaging, SMS.
It does not cover faxes or voice telemarketing.

Ministry of Internal Affairs and Communications(MIC) is responsible for enforcing the Act on Regulation of the Transmission of Specified Electronic Mail and actively works to fight spam in Japan.
MIC has vigorously enforced the provisions of the Act on Regulation of the Transmission of Specified Electronic Mail, sending a strong message to senders of commercial electronic messages that their activities will not be tolerated.

This website provides Japanese anti-spam materials.