Tax Question for Freelance Work (as a foreigner on a work visa) in Japan

Hi everyone,

I'm an individual who is currently living in Japan on a work visa. I'm a South Korean citizen and I occasionally upload YouTube videos as a hobby.

Recently, I was fortunate enough to receive a sponsorship offer from a company. I filmed a video for them and received payment for the advertisement.

This is where I need your help.
I'm not sure how to handle the tax declaration for this income. I'm hoping to get some advice from anyone who has experience with freelance work as a foreigner in Japan.

Here are a few specific questions I have:

1. I've already received the payment, and taxes were deducted in South Korea before the money was sent to me. Do I have to pay taxes on this income again in Japan? I'm concerned about double taxation.

2. Since this is income from a source other than my main employer, are there any special procedures or applications I need to complete in Japan?

3. Are there others here who are living in Japan on a work visa and earn occasional income from overseas freelance work? How do you handle your taxes and legal obligations?

Any advice or guidance would be greatly appreciated. Thank you in advance!