Side Job and Tax

Hello all.
Recently, I am looking for extra work which I have worked as full-time employee. From my current full-time job has no issue and no clause for side job. But still do not want my employer know about my side job.

Couple days ago, I got an offer for side job but the employer will handle the tax process and they also would like cover and deduct other tax and insurance including with pension.

I have several questions and concerns such as:
1) Will my full-time employer recognize my side job based on the withholding tax ?
2) Should I declare additional income on my tax adjustment on every end of year ?

Really appreciate for your comments and suggestions. Thank you.

1) Will my full-time employer recognize my side job based on the withholding tax ?
They may notice the difference.
2) Should I declare additional income on my tax adjustment on every end of year ?
If your side job income exceeds 200,000 yen per year.

Your company will file your usual salaried income for you and you will received your 源泉徴収票/げんせんちょうしゅうひょう (withholding tax statement).

With that withholding tax statement, you can/need to file tax return separately on your own together with your side income details. Or as you said your side job offer to file tax return for you. Just give them your withholding tax statement.

Thank you for your answer. Really help me a lot.

However, my 2nd employer will not file the tax refund for me, but he will file the same withholding tax as my 1st employer. In my case, will it be a problem with this?

It depends on how much you earn from your side job, but if it goes over a certain amount (I believe it was 200,000 yen in a year) you’d need to do the tax filing yourself (assuming your first employer categorizes the pay to you as “salary”).

In that case, you obtain the “gensen choshu hyo” (withholding tax slips) from both employers, get a form from the tax office, and do the tax filing on your own. (They both deduct taxes, so you need to declare your total income and pay taxes on the total.) You do it during the period of mid-Feb to mid-March for the previous year (Jan - Dec).

When it comes to pension and other social security systems, you should not be registered with two systems - so if your first employer has you enrolled in their social security (health insurance, pension, etc.) you need to tell the 2nd one you don’t need theirs.