Japan Travel SIM cards available from Amazon.co.jp
Softbank
Prepaid SIM for TRAVEL by Softbank 
"Prepaid SIM for Travel" by Softbank is the only prepaid sim card sold by a major carrier. Others are all by docomo's MVNOs.
As Softbank utilizes a so called "Platinum Band" which is Band 8(900 MHz), Softbank customers are supposed to be able to enjoy unthrottled data usage even during lunch and evening hours when there are many users at one time.
Softbank Shops are found here.
There are many more travel SIMs available in Japan. If you have a good command of the language and a Japanese address, you can have it shipped to where you are staying.
Traveler's SIMs are usually called "Prepaid SIM" to separate them from the ones for Japanese residents. Prepaid SIMs don't require an ID to activate. (But I have seen a "prepaid" sim at convenience stores which was meant for trial use for residents. It's sort of confusing....)
The SIMs for residents in Japan require a government issued ID and a Japan-issued credit card for a month to month contract, so please be careful.
IIJmio
Japan Travel SIM 1GB for up to 30days
This is one of the more well known brands for prepaid travel sim cards in Japan. They provide information in foreign languages such as English, Chinese, Korean, and Thai on the web. Also customer service is available in English and Chinese.
Supported Network: NTT Docomo(Japan's largest Network)
Standards and Frequencies: LTE: Band 1 (2,100 MHz) / Band 19 (800 MHz) / Band 21 (1,500 MHz) / Band 3 (1,800 MHz; only available in the Tokyo, Osaka, and Nagoya metropolitan regions) 3G: Band 1 (2,100 MHz) / Band 19 (800 MHz)
for the details, click here.
JP MOBILE
JP Mobile JAPAN Prepaid SIM card
I have used this SIM a few years ago and had a good experience with it. It is a nano SIM, but comes with adapters and a pin. Easy to connect and use.
You can choose out of 8days, 15days, 31days, and 91days.
Japan Prepaid SIM (1GB Data for up to 15 days) $21.99+tax Free Shipping with Amazon Prime
