If you’re used to buying SIM cards in Asia in other countries, you’re going to be surprised at the prices in Japan. Like majority of things in Japan, they’re more expensive, but the quality is admittedly great. If you want to rent a SIM card or a mobile wi-fi router in Japan, I’ve had a good experience with SoftBank. It’s recommended to reserve on their website at least a few days before your trip, but I’ve had mild success just randomly visiting their kiosk at the airport.
You can pick-up and drop-off rentals in different cities, and they have kiosks in several locations.