Starting a Side Job in Japan While You Have a Full-Time Employer

Quick answer

Hi, I'm Miya, and I help newcomers settle into life in Japan. Someone offers you a weekend translation job, or a friend wants to pay you to build them a website, and the question in your head is simply "am I allowed?" It isn't one question. It's three, decided by three different parties: your employer, through its work rules; the immigration authorities, through what your residence status permits; and the tax system, through what you have to declare. A yes from one is not a yes from the others.

The check people skip is almost always the second. They ask HR, get a friendly nod, and assume that settles it — but a company has no authority over immigration rules. Below I'll take the three in the order I'd do them. This is orientation, not legal, immigration or tax advice: for anything that turns on your own situation, the National Tax Agency and the Immigration Services Agency are the sources that actually govern, and a licensed professional can read your case.

Start with your own company's work rules

Whether you may take outside work is set by your employer, not by a single national rule — which is why the answers you hear from friends contradict each other so wildly. Some companies prohibit secondary work outright without prior permission. Many run a permission system: you submit a short application naming the work, the client and the hours, and you get a written answer. A growing number allow it openly, since the government's model work rules were revised years ago to treat outside work as permitted in principle rather than banned by default.

So go and read the document. It's the shuugyou kisoku, the work rules, usually on the HR intranet or available from HR on request, and the relevant clause often sits near the ones on confidentiality and conflicts of interest. You want to know whether permission is required, who grants it, and what conditions come attached. Those are predictable: no competitor work, no company equipment or confidential information, nothing that interferes with your duties or your rest.

My advice, plainly: ask before you start, and ask in writing. A one-line email to HR that gets a one-line reply costs nothing and leaves you a record. Explaining afterwards is a much worse conversation — and as the next section shows, the company often finds out anyway.

Your residence status is a separate question entirely

Here's the part that catches people. A residence status doesn't simply permit "working" — most work-related statuses permit a defined category of activity, tied to the job you were granted it for. Paid work outside that category can require permission to engage in an activity other than the one your status covers, the shikaku-gai katsudou kyoka, applied for at immigration. Whether you need it depends on your status and on what the side work actually involves, and it is not something your HR team can grant, waive or judge for you.

Statuses based on who you are rather than what you do — permanent residence, spouse or child of a Japanese national, long-term resident — generally carry no restriction on the kind of work you may do. Statuses based on a specific professional activity are the ones to check carefully, because getting this wrong can reach your residence status itself.

Take it to the Immigration Services Agency's own guidance, to a foreign-resident consultation desk, or to an administrative scrivener who handles visa work — before you accept the job, not after the first payment lands.

Side income tends to surface through resident tax

People assume side work is invisible to their employer. Usually it isn't, and resident tax is why. Your municipality assesses resident tax on your total income for the previous year, then tells your employer how much to deduct from each month's salary: special collection, tokubetsu choushuu, automatic for most employees. The figure is recalculated once a year and the new amount appears on pay slips around early summer. If outside work pushed your total up, that deduction goes up too, and the person in payroll reading the municipal notice can see it doesn't match your salary.

When you file a final tax return, there is generally a choice on the form about how resident tax on income other than employment income is collected: through your salary, or by ordinary collection, futsuu choushuu, where the municipality bills you directly. Whether that option is open for your type of income varies, and municipalities handle it differently, so ask your city or ward office rather than assuming. One honest limit: if the side work is itself an employment relationship — a second payroll job — that wage income generally still flows through the main employer's deduction.

And treat the choice as a matter of collection routes, not concealment. If your company's rules require permission, the fix is to get permission, not to route a tax bill around the payroll office.

Above a certain level, you file the tax return yourself

With one employer, your income tax is normally settled by the year-end adjustment, the nenmatsu chousei, and you never file anything. Once money arrives from outside that payroll and passes the level set in the rules, you generally become responsible for your own final tax return, the kakutei shinkoku, in the following year's filing season. I'm deliberately not quoting the threshold: it depends on the type of income, it isn't the same test in every situation, and the National Tax Agency publishes the current rule. Your tax office will also answer questions in person, free.

The habit that makes this painless is boring and starts on day one: record every payment with its date and payer, keep the invoices or platform statements, and keep receipts for what you spent to do the work. Check each statement for tax already withheld at source, the gensen choushuu, because that has typically gone to the tax office already and feeds into your final calculation. Reconstructing all of it in February from bank-transfer lines is miserable, and I've watched people try.

Income is still income if the client is overseas, if a platform takes its cut first, or if it arrives as goods rather than cash. And if the work keeps growing you eventually cross into different territory — registering as self-employed, and the qualified invoice system — which we cover in separate guides.

Sorting the line all of this runs through

One quiet practicality sits under all three checks: side work in Japan is arranged, verified and paid through a phone number. Freelance and gig platforms confirm your account with a code by SMS, and many won't complete registration with an overseas number. A new client asks for a contact number before the first job. Payment and banking services send a verification code before they'll release money to you.

So I'd sort the line out before you start saying yes to work, rather than halfway through an onboarding form. Applying is refreshingly simple: you apply online with your residence card, and eSIM-capable phones can often connect the same day, typically in about fifteen minutes. Have a look at our setup guide for eligibility and the steps, and confirm the current terms on the page before you apply. Then work through the three checks in order — work rules, residence status, tax — and take the side job with nothing hanging over it.