How to get a data only SIM card in Japan
BIC Camera, an electronics retail chain in Japan, sell prepaid data only SIM cards under the name BIC SIM. Prepaid ("pay as you go" in the UK) SIM cards don't seem to be very popular in Japan as the major Japanese mobile phone networks like docomo, softbank and au don't offer them.
I recommend prepaid SIM cards to anyone who has an unlocked phone, is uncertain about the length of time they'll be in Japan, and wants to avoid paying their contract out in full for leaving early.
The initial SIM card loaded with 500MB costs ¥4,000 (approx £22) and can be recharged with either 500MB at a cost of ¥1500 (approx £8) or 2GB at a cost of ¥3000 (approx £16).
There is a BIC Camera shop in Shinjuku [Map] where you can purcahse the SIM. If I remember correctly the mobile phone area is on the second floor.
Make sure you bring your unlocked phone and get a member of staff to call up and register your SIM.
That's all there is to it really. Once your 500MB has been used up you won't be able to use the internet. Just return to the store and purchase a recharge pack and get a memeber of staff to recharge if for you.
If you have any questions send an email to firstname.lastname@example.org.