"Rocket Mobile" offers a "God Plan" with a monthly limit of 298 yen [excluding tax] and no capacity limit, although it is slow at a maximum download speed of 200 kbps.
The MVNO service was launched today in 2016.
200kbps is a tough speed for regular use as a main line, but it is an attractive plan for IoT devices and a second smartphone line, as long as it is connected to the Internet regardless of the speed.
It is unique that you can get points by installing questionnaires delivered by affiliated partners and installing the introduced application, and you can use these points for payment. If you earn points, you can use it for free.
Initially it was only data communication, but later it also supports calls. Now, you can also select the SoftBank line and au line. However, there is no call plan for SoftBank lines.