The voltage used in Japan is 100 Volt, which is different from North America (which is at 110V), Central Europe (which uses 220V) and most of the other various regions of the world. Japanese electrical power plugs will almost always have two, non-polarized pins. However, they do fit into North American outlets.
Japanese electricity and Japanese foreign power outlets are identical to the ungrounded (2-pin) outlets found in North America. While many of the Japanese outlets these days are polarized (meaning one slot is slightly wider than the other), it is possible to encounter non-polarized outlets in some places still. So a plug adapter should not be necessary in the vast majority of locations, unless you are traveling to somewhere “off the beaten path.”
Some North American equipment will work fine in Japan without an adapter or foreign converter and vice versa, however, some sensitive equipment may not necessarily work properly or it may even get damaged so it is strongly recommenced you obtain a foreign electricity converter.
The frequency of the power current is 50 Hertz in Eastern Japan (which includes Tokyo, Tohoku, Yokohama, and Hokkaido) and it is 60 Hertz in Western Japan (which includes Nagoya, Kyoto, Osaka, Hiroshima, Shikoku, Kyushu). Note however that this frequency difference affects only sensitive equipment.