My Number

This page captures some of my thoughts about the Japanese My Number (個人番号) system.

The law that set the path for the My Number system in Japan is called 「行政手続における特定の個人を識別するための番号の利用等に関する法律」 which I will call My Number law for the rest of this article. The My Number law first came into effect in April 4, 2017. The most recent version of the My Number law came into force on December 12, 2025.

This article presents concerns from a technical and privacy point of view. This article does not present a legal analysis and I'm not a lawyer.

One number for everything

第一章　総則

（目的）

第一条　この法律は　…　個人番号　…　の有する特定の個人　…　を識別する機能を活用し、並びに当該機能によって異なる分野に属する情報を照合してこれらが同一の者に係るものであるかどうかを確認することができる…。

The idea of the My Number law is to assign a unique number to every person that you can use to tell persons apart. The 厚生労働省 and 法務省 intent to use this system as a unique identifier that many other public and private systems can attach personally identifiable information (PII) to. Here's are some examples of PII that currently is connected to someone's My Number:

• Name
• Date of birth
• Gender
• Address history
• Electronic Health Records
• Immigration and residence status
• Personal income and tax information
• Information about family members
• Bank account information

Problem

Any party that knows someone's personal number and has access to the data repositories that store any of the above records can access someone's PII. Apart from legit use cases, the major concern here is that if someone has access to data repositories that they should not have access to, they can gain sensitive information about a person without that person's knowledge.

Linked to a physical card

Several government ministries urge citizens to apply for a physical My Number Card. This My Number Card shows the following sensitive information about a person:

• Full name
• Current address
• Date of birth
• Expiry date
• Previous home addresses
• Personal number

The My Number Card further is equipped with smart card functionality that lets you conduct various official businesses using its integrated digital certificate. With this digital certificate, you can sign or identify yourself. The smart card is protected by several passcodes:

• 住民基本台帳用: Passcode that allows you to change information about you on the card when going to town hall. (4 digits)
• 券面事項入力補助用: Passcode that lets you electronically transfer address, name, and other information about you to other devices. (4 digits)
• 利用者証明用電子証明書用: Passcode that lets you identify yourself when authenticating with the マイナポータル, when printing your registration information (住民票), or when going to the hospital (4 digits)
• 署名用電子証明書用: Passcode that lets you use the digital signing feature in your card. Example: Personal tax filings when using the e-Tax system. (6-16 digits)

Problem

• Theft or Loss: Losing the physical card means that anyone can see all of the sensitive information printed on the card.
• Single point of failure: Losing this card means that you can't conduct any of the official business in the preceding list.
• Single point of failure (medical): You can't prove that you have health insurance if you don't have your My Number Card

Unchangeable number

There are no provisions for changing your personal number.

Problem

• Your PII accessible in eternity: If an unauthorized third party learns your personal number and can unlawfully access any data repository that links your personal number to sensitive information, they can access this sensitive information in eternity.

Passcodes written on paper

When you apply for a My Number Card, you need to set the four passcodes as described in #Linked to a physical card. At Kitazawa Town Hall (北沢総合支所 ) in Setagaya City, town hall employees asked me in 2023 to write down my passcodes on a piece of paper and hand it to them. The employees would then set the passcodes themselves and give me the card.

Problem

• Third party can learn passcode: If the pass codes are not shredded properly or if an unauthorized party somehow gains access to the pieces of paper with passcodes on them, they're able to conduct most relevant official business against your will - if they're able to retrieve your card as well. This is an unacceptable risk.
• Town hall employees know your passcode: Since you're instructing town hall employees to set your passcode, they know your passcode. They can conduct official business against your will. If you don't change your passcode immediately, they may wait for a few months, retrieve your card (for example when visiting town hall a second time), and use the card without your authorization. The fact that it's possible at all is concerning.
• No good passcode hygiene: There are no instructions on how to choose a safe passcode that is difficult to guess or brute force. While the My Number card locks you out after a number of unsuccessful passcode entry attempts, you can just set your birth year as a passcode and someone can guess this passcode by looking at your date of birth written on the card that they've just stolen from you.

Other concerns

• Misinformation: 厚生労働省 deliberately does not tell citizens that they can use their health insurance without a My Number card, for example by obtaining a 資格確認証, a plastic card without any electronic functionality. I personally only know a few people that know this. And they're the ones that told me.
• Waste of tax money: If you visually compare the 資格確認証 and 健康保険証 issued by 協会けんぽう you will notice that they mostly look the same. One's in yellow, and the other one's in blue. To think that the government spent 10 years rolling out the My Number health insurance card scheme, and a yellow plastic card offering all the functionality that you need, shows that the My Number health insurance card scheme is not effective. When you consider that after all of this busywork, I get another card that looks the same as my old one, you may think: Why didn't they just keep the card blue?
• Usability: To use any of the electronic functionality, you need to be at least somewhat competent at using IT systems. If you're not good at it, you can't do the following:
  • Go to the doctor
  • File your taxes online
  • Conduct any other official, electronic business
• Reliability of My Number card in hospitals: For various reliability reasons, or if you forget your passcode, patients can be asked to pay the full amount for medical treatments on the spot. You then need to apply for a refund from your health insurance. Almost 70 % of hospitals recently reported technical difficulties.
• Certificate expiry: If you forget to renew the electronic certificate on your my number card, you can't go to the hospital anymore or have to pay the full amount yourself. Again, for older people this presents a problem.
• Low adoption: Less than 50% of citizens use the My Number health insurance card scheme