If you like to stay connected to the World Wide Web while in Japan, getting Wi-Fi is probably the most secure option.
Japan Wireless offers various routers with different contracts. Depending on your desired speed, coverage and battery length, choose the one which suits you best. Base rate goes up to 4 days, after that you have to pay extra. Japan Wireless also offers prepaid SIM cards. Make sure you order (a few days) before going to Japan.
More info on: japan-wireless.com
Photo courtesy of japan-wireless.com