I want to emphasize what Oklah! said because it's something I had a hard time understanding back in the days.
TL, DR:
- permanent tax resident is not the same as your "permanent resident" status on your visa. The two are uncorrelated. You are considered a permanent tax resident if you have lived in Japan for 5 years or more in the past 10 years.
- as a permanent tax resident, you have to declare your income worldwide. It does not matter that the investment is in a Singapore brokerage account
- Depending on your situation, declaring your income does not necessarily mean you will have to pay tax on it to Japan, but in my experience that is the most likely outcome. Tax treaties ensure you don't get double taxed, but if you paid 0% in Singapore it just gives you a foreign tax credit of 0 yen in Japan, meaning you'll probably have to pay the full bill to Japan.
- Japan has an exit tax for people who try to game the system (e.g. go live one year abroad, sell all your investments there to realize the capital gain at no tax, then come back to Japan)
Paraphrasing:
If you're considered a permanent resident of Japan from a tax perspective (which is different from the "PR" visa, and depends solely on how long you've been in Japan), you have to declare all your income to JP tax office, independently of where in the world you made that money.
Of course if you already pay taxes on that money in, e.g. Singapore, you might be able to claim tax credit in Japan (depending on treaties between the two countries), but my limited experience with that is that, although you don't get double taxed, you always end up paying the maximum of the two countries. E.g. if Singapore has a 0% tax on some category of income and JP is 20%, JP will say "ok, you get to pay us 20% minus the 0% you already paid, so you just owe us 20%
The situation is possibly different if you have been in Japan less then 5 years so far. So maybe what you suggest is both legal and advisable for your first 5 years of Japan, but of course you should absolutely talk to an accountant to confirm this, otherwise you could be hit with some massive tax penalty in the future.
Also note that to prevent people from gaming the system, Japan also has an "exit tax": if you're a permanent tax resident with significant worldwide assets (approx 1 million USD or more) and leave Japan, you will have to pay a tax on unrealized capital gain from your worldwide investments, as if you had sold those investments:
https://www.pwc.com/jp/en/taxnews-inter ... 114-en.pdf