[tkgally]

> You would have to make the rounds yourself informing the governments concerned you renounced their respective citizenship.

The Japanese government requires that you have renounced your other citizenship(s) as a condition for acquiring Japanese citizenship [1, item 5, in Japanese]. I know a number of people who have taken Japanese citizenship, and they all had to go through a formal citizenship-renouncing process at the embassies of their previous countries of citizenship.

Exceptions seem to be made only in cases in which the applicant, for some reason, cannot renounce a previous citizenship.

[1] https://www.moj.go.jp/MINJI/minji78.html#a09

[toyg]

The Japanese government, however, doesn't go around checking other countries' citizenship lists (nobody does). I know of several Japanese citizens who accrued other nationalities but never informed the japanese government. Obviously they'd be stripped of their JP rights if it emerged, but until then...

[robjan]

They don't check citizenship lists but you must show proof of renouncing the citizenship attached to the passport you used to enter the country.

[toyg]

I'd expect them to ask for this only to the non-Japanese-looking, or people who acquired citizenship after birth so might be in some particular database. I have a friend who goes back to Japan fairly regularly and afaik they were never asked as such when entering with her Japanese passport.

[tkgally]

I heard of an interesting case. A red-haired friend of mine who was born in Russia and, after living in Japan for many years, took Japanese citizenship was detained and questioned when trying to enter Canada on her Japanese passport. The Canadian immigration authorities were suspicious because she didn't look Japanese to them.

She said that she has never had any trouble entering Japan on her Japanese passport.

[robjan]

This is my point. Japanese people can have multiple citizenships but in order for a foreigner to gain Japanese citizenship, they must renounce at least one of their other citizenships to fulfil the "single citizenship" criteria.