Even for a short stay in Japan, you will need mobile phone for many practical reason.
The prepaid type mobile offered by Softbank is very useful for a short stay.
Note that docomo doesn't have this service any more (before they had). AU has the prepaid phone service, but the financial situation of AU has been apparently not good since several years, so this seemed to be kind of dangerous, as you may want to use it for next several years.
First, you need to buy these
1. mobile phone (prepaid type) : the price depends on the type, but mine was 2500 yen or so.
2. prepaid card : 2 types, 3000yen and 5000yen.
To activate it, call "1400", then follow the instruction
1. Charge credit: just give the number of mobile card
2. Register free email service: this cost you 300yen, but this is cost-effective I think. this service is only for 30 days, so you have to do it again if you use more than the period.
Every time you come to Japan, you just have to buy the card and do the above.
The credit is valid for 2 months from the date you charged.
The number is valid for 1 year after the last credit is expired.
Voila.
