If you are staying in Japan for longer than 90 days, within the first 90 days of your stay you must register as an alien (foreigner). You can do this at your local municipal office´s Foreign Resident Registration Desk.
The alien registration process is relatively simple. You will need to bring your passport and a passport-size photograph with you and fill out forms declaring your status of residence, your address in Japan and other information such as age and name. If you are spending more than a year in Japan your fingerprints will also be taken. While you should apply in person, a family member is permitted to submit your application in the event that you are unable to do so yourself (due to illness, for example). Applicants under 16 years of age are immediately issued their alien registration cards (gaijin cards), while older applicants are given a date on which they can return to pick up their cards.
Your alien registration card shows all your basic information and serves as your personal identification while in Japan. If you are 16 or older you must carry your alien registration card with you at all times. Until you receive your card, carry your passport as identification.
Adult alien registration is valid for a period of five years. After that you will be required to renew your registration. Foreigners under 16 are required to renew within 30 days of their sixteenth birthdays. Anyone who originally registered through a family member will have to renew within four years (the date will be specified when their initial registration is processed), and they will be fingerprinted at the time of renewal. Finally, those whose status of residence is unconfirmed or who are extending their residence after less than a year must renew their registrations after one year. Renewal works the same as application.
If you lose your alien registration card you should notify your local police immediately. You then have two weeks to apply for a new card.