Another provider of portable Wi-Fi networks is E-connect Japan. The cheapest routers here go for 420 yen a day, and depending on the length or your rent the price can go down. E-connect also offers prepaid SIM-cards. The return of both the Wi-Fi router and the SIM-cards is extremely easy: just put everything in the envelope provided at the end of use and drop it in any mailbox in Japan!
Photo courtesy of econnectjapan.com