While staying in Japan, you'll need to understand its currency at some point. Japanese Yen (cash) is the only acceptable payment type in most of places except a few popular sightseeing locations. Many stores and restaurants don't accept credit cards especially in rural areas, so getting cash is the key to avoid any trouble about money during your visit in Japan. You can withdraw, transfer or exchange money in most convenience stores in town with your card, so don't worry too much even if you forgot to exchange currency before your trip. Converter apps ;ike XE are a great tool to see how your money is flowing.