If you're going to be in Japan for under 14 days, the following are your choices. They're about 3,000 to 4,000 yen.
Prepaid SIM for Japan (NTT Communications) http://www.ntt.com/personal/services/mobile/one/visitor/en/store.html
b-mobile VISITOR SIM (b-mobile) http://www.bmobile.ne.jp/english/index.html
BOJ mobile Prepaid SIM (Beauty of Japan) http://www.prepaid-sim-japan.com/
U Mobile Prepaid (U-Next) https://umobile.jp/lp/prepaid_eng