There are 2 types of pension system in Japan.
Standard government pension “Kokumin Nenkin” which the contribution is currently fixed at 16,490 / month.
There is also another type of pension Employees Pension “Kosei Nenkin” which is part of social insurance “Shakaihoken” for full time employees.

The pension can be received under different circumstances:
-Reach 60 years old
-Become disabled and cannot work
-Are the dependent spouse of someone who passes away

If you have been enrolled for 40 years (20-60 years old) you are eligible to receive the full pension.
The current amount (2017) for government pension is 65,008 / month.
For Employees pension it is 100,165 / month.

Obviously this is not enough to live on but provides some security.
Joining the pension system is compulsory in Japan.
If you do not pay it is difficult to apply for an home loan and the government may seize your assets.
If you do not enroll immediately you will have to backpay the previous 2 years of payment.

I believe the best thing to do is pay the pension as it is required and continue to save and invest indecently for your own future.
You do not want to be dependent on the government for your financial stability.