Once you arrive in Japan, turn on and set your mobile phone network to SoftBank (or 44020 or JPNJP). Your mobile phone is now roaming capable anywhere in Japan.
Access the network settings on your handset to select your mobile network to SoftBank.
SoftBank is W-CDMA (3G) network. If your dual mode handset is set to automatically select GSM, change dual mode settings to W-CDMA.