Tor has been strengthening itself from this sort of blocking for a long time now. Japan is late in the game, and I doubt they are going to convince ISP's to block with the intensity of the Chinese censors, who are currently the most skilled at trying to block Tor. Right now it is give and take between Tor and China, with neither of them staying ahead for too long. With the advent of obfsproxy Tor has been making serious progress in their blocking resistance though. Their bridging system can still be significantly improved, but they generally don't want to improve it until it is required to (they actually like the back and forth, and prefer to keep the progress of the blocking technology as slow as possible, rather than risk getting in a fast arms race that they might end up losing). So, for one it is not likely that Japanese ISP's are going to be at the forefront of Tor blocking, and it is entirely possible that simple bridges will be enough to get around it. And for two, even if they match China in their blocking abilities, the general back and forth will still apply, and bridges + Obfsproxy version whatever will be able to circumvent them much of the time.