• There are 3 major mobile phone operators in Japan, NTTdocomo, SoftBank, and au/KDDI.

• Now only SoftBank has 3G/UMTS SIM card based prepaid service. (As of Feb 2009)

SoftBank 3G covers all major cities in Japan.

• There is no GSM/EDGE/GPRS service within Japan.

• Find details at SoftBank's web site (English).

You may have to buy the SIM card with a handset, at first.

I have never bought this type of SIM and handset, so please ask shop staff or customer call centre whether your own handset can be used or not with the SIM before purchase.

• APN: softbank

Username: ai＠softbank

Password: softbank

• Note: SoftBank is formerly vodafone Japan.

vodafone UK's SIM can be used in Japan via SoftBank network.
Maybe other vodafone's SIMs are as the same, but not for sure.

• You can find SoftBank shops in Narita Airport.

Terminal 1: 1F(ground fl.) and 4F

Terminal 2: B1F

• In Tokyo, at least SoftBank Omotesando shop has English speaking staffs.