PuPuRu WiFi Potable Wifi (Pocket Wifi) Rental Service
You can rent a WiFi router covering a large areas compatible with the KDDI network. Although they offer two plans (10GB / data unlimited), the 10GB plan will be sufficient for regular use of the internet if you stay in Japan less than 20 days.
Unlimited data, covering local areas!
KDDI is a parent company of WiMAX, and accordingly you can use the 4G LTE by au in the areas covered by WiMAX 2+, which is wider than that of by WiMAX. Also the downloading speed becomes faster accordingly.
If you go to local areas, the trouble of unconnected with the internet will be decreased as you can use WiFi via the network by KDDI.