Once in Japan, almost all matters associated with your visa can be handled at the Tokyo Regional Immigration Bureau in Konan, Tokyo (5-5-30, Konan, Minato-ku, Tokyo): http://www.moj.go.jp/ENGLISH/information/iic-01.html.
Most Japanese visas can be easily extended by filing the appropriate forms (which are in English and Japanese) at the Immigration Office. Documents for extending each kind of visa vary but generally include:
- your passport,
- alien registration card,
- proof of employment (most companies can provide a standard form you can give to immigration, otherwise your employment contract will often be sufficient),
- proof of tax payment (end of year tax stubs which are used for filing income tax returns).
Temporary Visitor or Tourist Visas can be extended in some situations, although an extension is not guaranteed. Immigration Officers may ask why you want to extend and you may be asked to show that you have the means to continue living in Japan including adequate finances and a place to stay.
Once your materials are prepared and you have filled out the appropriate application (as detailed above) the Immigration staff will direct you to the appropriate counter where you turn in your materials and the immigration team processes your request for extension. Once you have received your visa extension you will need to return to the Ward office that issued your alien registration card to have your registration updated. This must be done within 14 days of your receiving your new visa. More information on visa extensions applications can be found here: http://www.immi-moj.go.jp/english/tetuduki/index.html.