It's a law
The Japanese police can ask for your passport while you are walking on the street and if you cannot identify yourself you get fined & it can cost a LOT. So don't forget it.
It also comes in handy when you are shopping tax free. As a lot of products in Japan can be purchased tax free if you have your passport with you.
Vai a consultare la mia guida per le migliori cose gratuite da fare, nonché itinerari e consigli di viaggio per rendere il tuo viaggio indimenticabile.