Nowadays, you can use your credit card at many places in Japan. Still there are many other places where you need cash for your transaction. So, you should keep some amount of cash in Japanese yen with you. Here I suggest two ways of getting cash in Japanese yen:
(1) Find ATM at any branch of Japan Post Office. There you can use virtually every cash/credit card. Go to the link below for more details.
Cash Withdrawal at Japan Post Office
(2) Find ATM at any 7-Eleven convenience store. You can use virtually every cash/credit card there. Go to the link below for more information:
Of course, you can get cash in Japanese yen at both Haneda and Narita airports. Although I am not 100 percent sure, this seems to be a lot more expensive than the two ways explained above.