iDeCo in Japan's Year-End Adjustment: How to Claim

Last updated: 19 October 2026 · the National Tax Agency and the National Pension Fund Association · the 2026 tax year

What you pay into iDeCo, and your own matching contributions to a company defined-contribution plan (企業型DC), is deducted from income in full: the deduction for small-business mutual aid and similar premiums (小規模企業共済等掛金控除). If it comes out of your salary, your employer already counts it and you do nothing. If you pay iDeCo from your own bank account, you hand the contribution certificate in with the year-end adjustment (年末調整) forms. On a salary of ¥5,000,000, ¥20,000 a month gives back ¥12,200 of income tax and takes ¥24,000 off resident tax. How iDeCo changes each month’s pay is in the guide to iDeCo and your monthly take-home.

This is a general explanation for an employee. Your employer, your iDeCo provider and the National Tax Agency are the authority on your own case.

Paid through payroll: nothing to do

If your employer takes the contribution out of your salary — iDeCo paid by the employer on your behalf (事業主払込), or matching contributions to a company plan (マッチング拠出) — it is already in the monthly withholding, and the year-end adjustment includes it too. No certificate is sent for these contributions, and you do not write them on the form. Your employer’s own contributions to a company plan are neither your income nor your deduction.

Paid from your own account: the certificate

The National Pension Fund Association (国民年金基金連合会) posts a contribution certificate (小規模企業共済等掛金払込証明書). For 2026:

• 23 October 2026, by post (electronically from 21 October), if you paid anything from January to September. It shows what you paid to September and what is due from October to December.
• Later, if you started late: 25 November for a first payment in October, 22 December for a first payment in November, 22 January 2027 for one in December. A certificate also comes again in those months if the year’s total changed.
• Lost: ask the company that keeps your iDeCo records (運営管理機関) for a new one; it takes about two weeks. An electronic copy can be downloaded from e-私書箱 if you have an account.

Where it goes on the form

On the Application for Deduction for Insurance Premiums (保険料控除申告書), in the box for 小規模企業共済等掛金控除, write the year’s total from the certificate on the line for individual-type pension contributions (確定拠出年金法に規定する個人型年金加入者掛金). Cut out the page of the postcard marked 重要 (important) and attach it. The total includes October to December as amounts still due; if a December debit then fails, the deduction was too large and you correct it with a tax return.

If you miss the year-end adjustment

• Before the end of January: your employer may still be able to redo the adjustment; the guide to fixing a year-end adjustment covers it.
• After that: claim it on a final tax return (確定申告) with the certificate. A refund return can be filed for 5 years, counted from 1 January of the year after.
• A certificate that came in January — a first payment in December — usually misses the adjustment and goes on a tax return.

What it saves: two examples

¥20,000 a month (¥240,000 a year) paid from your own account, for somebody single, aged 30, in Tokyo, paid the same salary each month:

Resident tax is a flat 10%, so it falls by the same amount on both salaries; income tax falls by more on the higher salary, because more of its income is taxed at a higher rate.

Questions people ask

Do I need the certificate if iDeCo comes out of my salary?

No. Your employer already takes the contributions into account, both in the monthly withholding and in the year-end adjustment, and no certificate is sent for them.

I lost the iDeCo certificate. What now?

Ask the company that keeps your iDeCo records for a new one; it takes about two weeks. If you set up an e-私書箱 account, the electronic certificate can be downloaded from there instead.

Can I claim iDeCo on a tax return instead?

Yes. If it was not in the year-end adjustment, put it on a tax return with the certificate. An employee filing only for a refund can do this after the year ends rather than only in the February to March season.

Sources

• National Tax Agency — No.1135 小規模企業共済等掛金控除 (iDeCo and company-plan member contributions deducted in full; the certificate with the insurance premium form or the tax return). Japanese, as of 1 April 2026.
• National Tax Agency — 令和8年分 年末調整のしかた (page 28: contributions taken from monthly pay need no certificate; those you pay yourself are deducted from the insurance premium form). Japanese.
• National Pension Fund Association (iDeCo) — 令和8年分の小規模企業共済等掛金払込証明書の発行時期と対象となる方 (23 October, electronic from 21 October; 25 November, 22 December and 22 January 2027 for late starters; not for contributions paid by the employer; reissue). Japanese, dated 1 October 2026.
• National Pension Fund Association (iDeCo) — 小規模企業共済等掛金払込証明書に関するよくあるご質問（令和８年版） (the line on the form; the page to attach; about two weeks for a reissue; a failed December debit corrected by a tax return). Japanese.
• Income Tax Act (所得税法) 第188条 — e-Gov (contributions taken from pay come off before the monthly withholding). Japanese.
• National Tax Agency — No.2030 還付申告 (a refund return for 5 years from 1 January of the following year). Japanese.