If you are traveling to Japan and want to use charge your electronic devices, you may need a travel adapter.
AC power plugs and sockets in Japan is JIS C 8303 which is almost the same as US's NEMA 1-15. These types are called TYPE A. Therefore charging US electronic devices, you do not need any travel adapter in Japan.
On the other hand, most of the European countries use TYPE C(Euro Plug). Therefore you have to attach the travel adapter type A to your AC charger's plug.
Also watch out that the voltage in Japan is 100V and the frequencies are 50 Hz in the Eastern Japan and 60 Hz in Western Japan. European countries are mostly 230 V and 50 Hz. Therefore you might need a transformer too. Please check the AC adapter.
Japanese type A adapter or universal travel adapter can be bought easily outside of Japan but it is not easy to get in Japan. One of the hotels in Nagoya had a travel adapter already plugged into the socket which is very rare. I assume that this hotel has travel adapters in their rooms due to it belongs to a French chain.