I worked in Japan (Kobe) for a year and this was a big surprise to me upon my arrival. The post office ATMs will always take your non-Japanese cards, but many others will not. Even a afternoon-long visit to a large Citibank branch in central Osaka got me nowhere. I always recommend having some travelers checks on hand. Use your credit card where you can, but many places in Japan do not take them. People tend to use cash.