(You need a CDMA or TDMA cellphone to be able to use it in Japan.)
There are no GSM cellular carriers in Japan.
Japan does not have any GSM mobile networks and uses a different type of technology. Most mobile phones purchased outside of Japan do not work in Japan. It uses TMDA (PDC), CDMA, and WCDMA technology that is not neccessarly compatable with a phone purchased for use on foreign cellular providers' networks.
For more information on using mobile phones in Japan please check this site: euc.jp/misc/cellphones.en.html
If you've used either of these networks or roamed in Japan, please post about your experience and the cost. Feel free to ask questions.