Based on the current rules, which may change in the next 20+ years ??? :
You cannot take the money out of iDECO until retirement age, age 60 or later.
However, when you move offshore, you will no longer be able to contribute, as non-Japanese are not entitled to make voluntary Kokumin Nenkin contributions overseas, so the account will be 'hibernated'.
You can get the Tax Saving advantage if you have taxable income to offset, and it would definitely be to your advantage. The higher your income, and marginal rate of income tax, the more you save, and get the government to contribute to your iDECO pension.
As a regular employee (guessing), you would (probably) be entitled to invest Y23,000 per month, out of which the government will contribute around 40% in tax saving...
Definitely recommended (if you are not a US Citizen or Green Card Holder, due to PFIC), as the invested funds will grow gross, but will be taxable either in Japan, if you are still here, or overseas when you receive disbursements. You should check (probably Article 17 of) the Tax Treaty between Japan and your country regarding taxation of Pension income.
https://www.mof.go.jp/english/policy/ta ... st_en.html
It is a long way in the future, but you will probably be able to take a Tax Free (in Japan) Lump Sum and then receive an Annuity Income, which would both be taxable only in the other country where you will be living then.
You should also consider NISA.
You can take the money out of NISA at anytime tax free in Japan.
However, when you move offshore, the gains from NISA may not be tax free in the other country, so you would probably want to liquidate all NISA before leaving (probably in the year before the year in which you leave) and transfer Cash from Savings so as to avoid taxation at the other end.