The Japanese Family Register System (Koseki Seido)
In Japan, there is a family register system in which births, deaths, marriages and other personal related information are registered as official records.
Foreign nationals living in Japan should register births, deaths and marriages that occur in Japan in accordance with the Family Register Law (koseki ho).  Certificates of registration and any copies should be kept in a safe place as they provide proof of personal identification.
Births, deaths, and marriages should also be reported to the government of the individual’s nationality. For information on procedures, enquire at your country’s embassy or consulate.

This information is from the CLAIR website. The Council of Local Authorities for International Relations (CLAIR) was established in July 1988 in response to rising concerns about local level internationalization in Japan.