Tokyo's voltage is at 100v, ( not 110 v ) BUT the frequency works at 50 Hz.
US frequency operates at 60 Hz. If the internal wiring insulation of any of your electrical devices is cheap or old, ( most likely not B&O ) there could be problems.
Eastern Japan, ( Tokyo, Yokohama, Hokkaido, Tohoku ) operates at 50 Hz, while western Japan ( Nagoya, Osaka, Hiroshima, Kyushu, Shikoku, and onward to the southwest ) operate at 60hz. The wiring in your device WILL run hotter because the amperage will increase as a result from running a 60 hz device plugged into a 50hz power source. Sensitive equipment may not run properly, and even become damaged. If it is hot enough and the wiring insulation is bad enough, there could be a fire.
Note also that the equipment plugs are similar to US standard flat two-prong, but can either be polarlized or non-polarized.
Specifically, what equipment ( model and age ) are you considering to operate in Japan ?