If you activate international roaming, you can use your Smartphone in 3G service areas whilst in Japan.  Please beware of high international roaming charges.
Japanese SIM cards have for a long time really difficult for visitors or foreigners to obtain. Recently there are companies that are making it easier. You can find short stay 3-14 day pre-paid data only SIM cards at airports or convenience stores in Japan. Alternatively, our favourite, mobal, offer a pre-paid or monthly Japanese SIM with voice, text and data. With free shipping worldwide and free pick-up in Japan, you can be connected the moment you step off the plane.
Prior to mobal, we recommended CDJapan rental data sim-card and found it to be sufficient and affordable.