Ah yes, many outlets in Japan are only 2 prong ungrounded. I always travel with one of these adapters, as I have encountered several situations where I have an ungrounded outlet to plug my 3 prong computer power supply into. In addition to Japan, this is also very prevalent in Taiwan. Be very careful though, as you are defeating the safety ground of the device you're plugging in, and should it fail or become defective, *you* end up being the ground conductor (electrocution)! Have you ever felt an AC line voltage go through your body? Not nice, and for some it can be deadly!