My Number (マイナンバー) or Individual Number (個人番号, kojin bangō) is a 12-digit ID number issued to all citizens and residents of Japan (including foreign residents in Japan) used for taxation, social security and disaster response purposes. This is equivalent to Social Insurance Number (SIN) in Canada, National Insurance (NI) in the UK, etc.
Once you have already registered your Resident Record at your municipality office, you should receive a notification card — a paper card which notifies every resident of My Number (the Individual Number) — in the mail within three weeks. Please note that the notification card must be used with other photo identification documents, such as a drivers’ license or passport.
Once you have received the notification card, you may apply for an Individual Number Card or My Number Card ― a plastic card which contains an IC chip, and the bearer’s name, address, date of birth, sex, My Number (the Individual Number), and ID photograph, etc.
This card can be used as an identification card for identity verification as well as to receive a wide range of services including municipality services and electronic applications using the electronic certificate such as e-Tax.
For more details and information, please see https://www.kojinbango-card.go.jp/en-mynumber/