[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.