Originally Posted by
garykung
OP's son is safe for now. But the decision has to be made in less than 2 years.
Originally Posted by
numan
In the case of son trying to fly now, he acquired dual at birth but I too know of others (3rd hand) who have acquired after and maintain both passports without Japan knowing. It seems like a clear cut area legally that Japan doesn't allow it but lots of vague info on whether or not really enforced
Not true. The situation is very clear. If you were born after 1985 and have dual citizenship by birth (or more accurately before the age of 20), Japan may require you after the age of 21 to sign a statement when you renew your Japanese passport saying that you will "try" to give up your foreign citizenship, but it does not require you to actually do so nor in the case of most countries is it even possible. The U.S. specifically denies requests to renounce citizenship if the reason is a requirement by a foreign country against dual nationality. (Also BTW - if you apply for a Japanese passport renewal overseas, they generally won't even require you to submit the "I'll try to" statement.)
If you acquire foreign citizenship as an adult, and the Japanese authorities find out, they can and will strip you of Japanese citizenship although this is currently be litigated.