I stopped over Singapore before my Japan trip and this data-sim offer from Singaporean carrier StarHub seems to be good compared to what's in offer for Japanese local data-sim and MiFi rental offers:
http://www.starhub.com/personal/mobi...ernet-sim.html
For $32, you get a data-only dim with
- 2GB daily 4G Data
- Valid for 15 days
Data Roam fee-free to several countries, including
Japan :
Australia, Canada, China, Hong Kong, India, Indonesia, Japan, Macau, Malaysia, Myanmar, New Zealand, Philippines, South Korea, Taiwan, Thailand, United Kingdom, United States of America
Once the 15 initial days run out, top-ops are possible online (via Starhub app) by credit card, such as:2GB / day x 7 days for $25 SGD