For all information read attentively their website www.bmobile.ne.jp/english
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I came back from Japan and I can tell that its really easy to use "Bmobile" internet. But for me 1 GB was finished in one week. And I found out that its very easy to find special Travel SIM Card with internet in many Japanese electronic stores. I purchased mine on "BIC Camera" Yurakucho Store near Tokyo Train Station. "BIC Camera" has a lot of branches all around Japan. Here is the list of branches with maps from VISA News website - www.visa-news.jp/visitjapan/i-merchant. Also in their stores you find all kinds of electronics.
Have a god day and best travels!