You can join iDeCo.
If you are enrolled in defined benefit-type corporate pensions (defined benefit pension plan (DB), employees' pension fund, etc.), the maximum contribution amount is 20,000 yen. And, the combined amount with the amount equivalent to contribution amount in other pension plans such as corporate pensions (defined benefit pension plan (DB), employees' pension fund, etc.) and company contributions in the corporate-type DC shall not exceed 55,000 yen per month.