Registration of Foreign Residents
Within a short time (90 days) of your arrival, you will have to register in your municipality. Within a few weeks after filling out a few documents and providing a few pieces of identification, you will receive an ID card stating your current address in Japan, and other information, such as your name, age, etc. The municipal office will give you a receipt after you submit the required forms. This receipt must be presented to the municipal office when you pick-up your ID card. It's important that you not lose the receipt.
Driver's license
To drive in Japan, you need to either possess a Japanese or international driver's license. The International Driver's License which you have acquired before your arrival is valid for 1 year from your entry into Japan. However, the license must have been issued by an administrative office of a member country of the Geneva Convention. Depending on the country which the license has been issued, it may be invalid in Japan.
You cannot convert an international driver's license to a Japanese license but you'll be able to exchange an Overseas Driver's License for a Japanese License. A few documents will be asked for and a small fee has to be paid.
All the procedures are described here : Japan Automobile Federation