Nexus7 is an Android tablet developed by Google. I bought one of 2013 versions.
First of all, you must be careful that there are two versions of Nexus7. You should buy a Nexus7 which equips a SIM card slot, not Wifi version, to utilize mobile network anywhere.
(2) Rakuten SIM card
I got a SIM card provided by Rakuten, a famous internet company in Japan. Rakuten is also a Mobile Virtual Network Operator (MVNO). It offers several telecommunication, borrowing network from NTT Docomo, with low cost. The monthly fee is less than $10. Surprisingly, the speed is not so slow, 1.5MBPS of downloading data. There are several companies other than Rakuten, which provide similar services.
You must choose MicroSIM card, not NanoSIM one. NanoSIM is available to iPhone. Rakuten does not accept a request for changing the SIM card.
(3) Inserting SIM card
This process is quite simple. Be careful to treat the SIM card. If you touch the IC chip, it is probably broken with ease.
This part is a little confusing.
First, you must connect your Nexus7 to a Wifi network. If did not so, you may fail.
Some articles in the internet direct that you should turn off the Wifi and wait a couple of minutes after the initial settings. But in my case this procedure was not working. I called the setting to choose the mobile network setting. Then, I inputted several data according the instructing paper sent from Rakuten, to create a new APN protocol. When you made a contract of "Entry plus plan," you should input the APN name as "f1.mobile.ne.jp", instead of "lte-mobile.jp" if your Nexus7 is not applicable to LTE.
After inputting the data, perhaps you cannot save the setting. It is a bug occasionally occurs. You should try it again.
Finally, you reboot the OS sometime. I hope it will work well.