You can use your mobile phone in Japan in SoftBank Mobile or DOCOMO's 3G (3rd Generation) service area. All you have to do is bring your own SIM card and insert it to a rental phone or your own 3G handset.
For more details, please check with your local mobile phone service provider. Rental phone service is also available upon arrival at Narita Airport or Kansai Airport. For further information please contact any of the following companies.