I can't speak to Japan specifically, but I did work in Taiwan for 5 years and bumped up against this occasionally. I routinely "snuck" in ~$10K, but in googling it a bit it seems like this is stupid (and risks seizure). My advice would be to get the USD in Japan and declare it to US customs. Based on what I've read, you won't have to pay taxes on it or anything. As I said, though, I never did it that way. I just got the US cash and brought it in without declaring it.