Japan is a very cash-based country. I make sure to always have at least 10000 yen on me, to make sure I have enough dough to eat, catch a train and pay for any incidentals. I use my credit card when I'm shopping at a department store, international or national chain store (BIC camera, apple store) or paying for my hotel. I can't read kanji, but I found the ATM's located inside 7/11's have an English button, so that's where I get my yen from. Some places might accept debit, but I wouldn't know because I don't speak enough Japanese to ask.