If the person you're sending money to has a Japanese bank account then the easiest way to send money to them would be to do an international wire transfer to their bank account.

If the person you're sending money to has a U.S. bank account, and ATM card, you can deposit the money into their U.S. bank account and have them withdraw the deposited money from an ATM, though if large sums of money need to be transferred this method would not be sufficent, due to bank/atm withdrawl limits.

Another option would be to send an international postal money order. International Postal Money Orders can be obtained from the post office.