I wouldnt use a card, credit or debit, in japan if i can help it. Of my 19 exchange mates who had cards none of them could use them except in large citibank branches, not in postoffices, not in hotels, was a real pain in the ***.
Me on the otherhand, took 3 months worth of cash with me and got some more mailed everyonce in a while. You get a better exchange rate that way too.