In the countryside, I'd suggest Docomo's service because of their area
coverage.  For a short stay, there are rental services like -
http://www.globaldata.jp/

 o 3G-wifi router
  - supports upto 5 wifi clients
  - \1,580JPY/day (include Docomo 3G sim and all communication charge)

 o 3G usb dongle
  - supports Windows(XP or later)/MacOS(10.3.9 or later)
  - \1,280JPY/day (include Docomo 3G sim and all communication charge)

b-mobile provides prepaid/rechargable 3G USB dongle, and it's a MVNO
service using Docomo 3G network.  This is good for a long stay as well.
- http://www.bmobile.ne.jp/english/3g.html

 o b-mobile Doccica
  - \14,800 per dongle
  - includes 500 minutes to be used over 90 days

 o b-mobile 3G
  - \39,900 per dongle
  - includes 150 hours to be used over 480 days