[Kappabashi]

Japan doesn't use the GSM standard but one called PDC, which is only used in Japan. But they also have a 3G network, which can be used with UMTS enabled phones.

To use this network, your home carrier needs to have roaming contracts with the Japanese provider, sometimes you have to call your carrier to unlock the roaming feature.

Data transfer is also possible on this network. But be aware, data roaming can be INCREDIBLY expensive, something like $10-15 per megabyte you download. Some carriers offer international data roaming packages, where you can buy an amount of data transfer in advance, at lower rates.

If you're in Japan for a while or visit often, you could explore the possibility of getting a data card and contract in Japan. I don't now if this is possible for non-residents but it would come a lot cheaper that way if you use mobile internet a lot during your stay.