My Number

My Number (マイナンバー), officially "Individual Number" (個人番号 - kojin bangou), is a unique number assigned to every resident of Japan for tax and social security purposes.

The 12-digit My Number also acts as a Tax Identification Number (TIN) internationally^[1]. Those with savings or investments outside of Japan will ordinarily be required to declare their residency in Japan and their Japanese TIN.

Physical Card and Electronic Certificate - Durations of Validity

For foreign residents, other than Permanent/Special Permanent residents, the expiration of your My Number Card is same as the expiration date of your status of residence. Even if the expiration date of your status of residence has changed due to being granted an renewal, etc, that information will not be automatically reflected on your My Number Card.

Physical Card

Conditional of valid residency as noted above. The validity period for a My Number Card is 10 years (5 years for persons under 20 years of age). The actual expiration date of the physical My Number Card is your 10th birthday (5th birthday for minors) from the date of issue.

Electronic Certificate

The validity period for the Electronic Certificate held within the card is your 5th birthday from the date of issue/reissue.

Renewals

A few months prior to a physical card or electronic certificate expiring you will receive written notification with instructions how to apply. Be watchful for unlawful requests to renew cards and certificate.

Your PIN number associated with the card does not expire during the validity of the card.

Mynaportal and taxes

Beginning from the 2021 tax year, much of the data from your Mynaportal account can be passed on to the National Tax Agency's online return-preparation site and automatically used to populate items on an e-Tax return. The government refers to this function as "Identity Linkage" on its English website, and all procedures currently (April 2022) need to be done in Japanese, but the function promises to make filing tax returns significantly less of a burden for those able to file their taxes online.

As long as the service provider has registered (no problem in the case of the national pension and national health insurance), automatic entry has recently been expanded to include information for the following six items on your tax return:

Furusato nouzei
"Specified" brokerage accounts (kabushiki no tokutei kouza 株式の特定口座)
Home loans
Life insurance
Earthquake insurance
Medical-care costs (this makes My Number-based treatment records available beginning from September 2021, and is supposed to be fully functional for 2022 income-tax returns)

Procedures

Obtain a My Number card and register to use Mynaportal (the card must be used to log in, either from a PC with a connected card reader or a compatible smartphone).
Once logged in to Mynaportal, choose the external site you want to connect to from "motto tsunagaru" (もっとつながる).
Follow the instructions to set up the connection.
From your next login, you will be able to make use of that external site from within Mynaportal.

Advantages for filing taxes

Data obtained from your Mynaportal account is automatically entered into your online tax return.
Records for the data do not need to be kept by the taxpayer as long as the return is submitted via e-Tax.
Identity Linkage is persistent and connections only need to be set up once.
The NTA plans to expand the system in due course to pension withholding slips and other tax-related items.

Mynaportal limitations

Data is only provided by registered entities (here is the complete list as of April 2022).
Data for medical expenses has only recently begun to be made available and may be incomplete.

Footnotes

↑ Information on Tax Identification Numbers (PDF), The Organisation for Economic Co-operation and Development (OECD)

[1] Information on Tax Identification Numbers (PDF), The Organisation for Economic Co-operation and Development (OECD)

[1]