Japan and the US are not so different in this regard. Japanese plugs are mostly ungrounded and non-polarized, so you can use them in American outlets without issue for the most part (I have heard that certain appliances can act funny across the ocean due to the slight difference in voltage, but I haven't noticed any such problem with my appliances). You might need an adapter (note the difference between adapter and converter) to go the other way, as polarized or 3-prong plugs will not fit into most Japanese outlets.