There are various types of sim cards in Japan.
Both a lot of companies to sell and plans are various.
Meanwhile, I introduce the sim card which I recommend.
It is a prepaid SIM card of the companies called FREETEL that I recommend.
・SIM name：Prepaid Data SIM for Japan
・SIM Types：Micro, Nano
・Plan：7days 1GB 1,780 JPY ・30days 2GB 2,780 JPY
・Recharge：500MB Recharge Coupon 1,480 JPY
The data capacity is high-speed communication. After having used it up, you can use it at low speed of 150Kbps.
The reason I recommend this SIM is that it is low price and has unlimited use of Facebook, Wechat, WhatsApp, Kakaotalk, Line data.
Because this SIM is not sold at the airport, it is necessary to buy it after appearing in the town.
There are many sale stores nationwide.
Yodobashi, BIC CAMERA, YAMADA,,etc
I think that it is good to use the free wifi at the airport and to do the net with this SIM after going out to the city.