Year-End Tax Adjustment (Nenmatsu Chosei) and Filing a Return in Japan
Updated: 2026-09-03
Quick answer
Hi, I'm Miya. Around December your employer probably hands you a stack of forms and asks you to fill them in quickly — that's the year-end adjustment, or nenmatsu chosei. It's the way your employer reconciles the income tax that was withheld from your paychecks across the year with the amount you actually owed, so most employees never have to file anything themselves.
For a lot of people that's the whole story: fill in the declaration forms, hand them back, and you're done. But in some situations — a side income, more than one employer, or certain deductions — you file your own return (kakutei shinkoku) in spring instead. Below I'll walk through what each piece is, and I'll keep pointing you to official National Tax Agency guidance, because this is an explainer, not tax advice.
What the year-end adjustment does
Through the year, your employer withholds a estimated amount of income tax from each paycheck and sends it to the tax office on your behalf. Because that withholding is only an estimate, by December it's usually a little too much or too little compared with what you actually owe once your personal circumstances are counted. The year-end adjustment is the step where your employer adds up your yearly pay, applies the deductions you've declared, and settles the difference — which often shows up as a small refund or extra deduction on a December or January payslip.
The point is that, for a typical employee with one job, this replaces filing your own return. You don't calculate anything yourself; you just give your employer accurate information through the declaration forms. If your situation is unusual, check the National Tax Agency's guidance for your specific case rather than assuming the adjustment covers everything.
The declaration forms and receipts to keep
Your employer asks you to complete a few standard declaration forms so they can apply the right deductions. These typically cover your dependents and family circumstances, and a separate form for insurance premiums — things like life insurance or private pension contributions — plus any other deductions you're entitled to declare. You fill these in with your own details; your employer can't know them otherwise.
For anything you claim, keep the supporting paperwork. Insurers and pension providers mail out premium certificates in autumn, and you generally need to attach or present these when you declare. I keep a simple folder from around October so nothing goes missing before the December deadline. The exact forms and which certificates are required can change, so confirm the current versions and instructions on the National Tax Agency's official pages.
Who's covered, and who files their own return
The year-end adjustment handles most salaried employees with a single employer who submitted their declaration forms on time. If that's you, there's usually nothing more to do. But several common situations fall outside it — for example, having a meaningful side income, receiving salary from more than one employer, or leaving a job partway through the year without a new adjustment.
There are also deductions that the year-end adjustment simply can't process, so you claim them yourself by filing a return (kakutei shinkoku) in spring. Medical-expense claims and first-year home-loan deductions are often-cited examples, and donations can be another. Because the rules on who must — or may benefit from — filing are detailed, look up your own case in the National Tax Agency's guidance or ask a tax professional; I can't tell you which applies to you.
The general timing
The year-end adjustment happens at the end of the calendar year. Your employer usually collects the declaration forms sometime in late autumn, runs the adjustment in December, and reflects any refund or extra deduction on the December or early-January pay. That's why the forms feel rushed — there's a real payroll cutoff behind them.
Filing your own return is a separate, later window: the general personal filing period runs in spring the following year. If you need to file, that's your chance to claim the deductions the year-end adjustment couldn't handle. Exact dates and any extensions are announced each year, so check the National Tax Agency's official schedule rather than relying on last year's calendar.
Keep your number active for the paperwork
Here's where connectivity quietly matters. If you file your own return online, the tax agency's e-filing portal and the My Number app verify who you are by sending a code to your phone or by pairing with a smartphone reader — and the same goes for many My Number services you'll touch along the way. A working Japanese number isn't a side detail; it's part of getting through the year-end paperwork smoothly.
So I like newcomers to have their line sorted before the busy season. With Rakuten Mobile you only need your residence card to apply, and if your phone supports eSIM you can be ready in about 15 minutes with no store visit. Check the official page for current conditions before you apply — then, when December's forms and spring's filing come around, your phone is one thing you won't have to worry about.
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