In Japan, there are lots of shops that sell prepaid SIM cards.
Among them, we picked up ones offering foreign language support.
Yodobashi Camera
They have a wide variety of prepaid SIM cards, and for buying the best one for your purpose,
they have "SIM Counter," which gives you an advice on it.
One of their largest stores is located in front of Akihabara Station.
Yodobashi Camera Multimedia Akiba Store
Bic Camera
Their flagship store in Ikebukuro is the biggest.
Meanwhile, Yurakucho store located in Ginza is very convenience and close to the Palace area.
Ikebukuro Flagship Store
Yurakucho Store
There are some shops and vending machines of prepaid SIM cards at airports in
Narita, Haneda, Kansai, Chitose, and Naha.