Ninja WiFi Potable Wifi (Pocket Wifi) Rental Service
NINJA WiFi is a rental service provider of portable WiFi (Pocket WiFi), which can often be seen on news or advertisement in Japan. They have a cooperation with lots of physical stores and municipalities so that you can use their service on various places. Although their prices are bit pricy, you can rend & return a portable WiFi for free at airports.
Unlimited data, covering local areas!
SoftBank is a parent company of Y!mobile, and they offer the network wider than Y!mobile's.
Available with the areas compatible with 4G LTE, and the data communication is available in the areas as wide as ones offered by docomo and au.
Some WiFi routers provided by SoftBank offer unlimited data services, and which are said not to be limited on the data much.
However, they offer only a few as a rental service for travelers.