You can use CIRRUS, plus, mc, visa logo cards - there are more but you could check Japan Post's website for the exact info - in post office and post office bank machines (called Japan Bank). Familiarize yourself with what they look like as there are many of them all over the place but there are also many others which won't accept foreign issues cards. Citibank and 7-11 also accept foreign cards. Be sure you have a 4-digit PIN and that you completely understand the charges for use internationally imposed by your bank and that you understand your daily limit as well.

In stores, restaurants and hotels credit cards are usually but not always accepted. Don't expect souvenir stands or noodle shops to take them.

thanks so much

I always call my ATM and credit card issuers to inform them of my travel dates - have never had a problem at Post Offices in Japan - after 7 trips....
Enjoy!

This page only repeats the information provided above in the previous two brilliant posts before me did but with pictures.

http://www.japan-guide.com/e/e2208.html

Aloha!

I agree, the best place to use an ATM card in Japan is at the Post Office ATMs.

Mastercard and Visa are widely accepted, not all stores will take credit cards but many do.

On a trip to an onsen north of Nikko a couple of years ago I ended up in a fairly tight spot. I'd planned on using a credit card to purchase my train ticket (Tobu, not JR) but when I got to the station at Asakusa found that I couldn't purchase it directly from the agent, but would have to buy it from a travel agent across the way, and the travel agent wouldn't be opening for another 20 minutes or so. So instead I used cash (that I'd been planning on keeping in reserve) for the outbound trip, assuming that I'd be able to get cash from an ATM in Nikko or use a credit card for my return trip by using a travel agent in Nikko. This turned out to be not nearly so easy as I'd thought it would be, and in the end I was able to get a cash advance from a bank ATM (which would not accept my ATM card) using my Visa card (and very curiously without a PIN or similar password). Had I not been returning to the US via NRT that afternoon I'd have had more options, but I'd scheduled my last day very tightly.

Having sufficient cash on hand would have made things a great deal less nerve-wracking.