From October 2015, a 12 digit social security and tax number ("My Number") will be delivered to all persons with a resident record (juminhyo).
Your "My Number" will be delivered by post in the form of a card to the address on your resident record.
Your "My Number" will be necessary for social insurance, tax and other procedures. Please keep it in a safe place and do not lose it.
For a detailed explanation about "My Number," please refer to the website of the Government of Japan.