The law is that you are supposed to choose by the age of 22 and if you choose your Japanese citizenship you can't keep your other citizenship. Apparently, though, (and we have heard this) it is common to just not say anything about the other citizenship and it's fine. In 31 years of having this rule, there have been no penalties awarded (i.e. removal of Japanese citizenship).
But this is a real issue for many people we know. In CAJ's school profile it lists the number of nationalities as 35, with 34% of the student body (464 students) as having dual passports. Not all of these are Japanese-non Japanese families but a high percentage are.
Here is a telling portion of the article:
Accepting it [dual nationality] officially would be very difficult politically, as Kono [a politician who tried to introduce legislation allowing dual nationality] found out in 2008, and touch on lots of sensitive issues related to nationality identity. It is pretty much politically taboo at this time.This isn't just an issue being discussed in Japan. As you know, globalisation is happening around the world:
Since 1960, the number of countries where citizenship is automatically lost after an individual chooses to acquire another nationality has halved to around 60 — or 30 percent of the nations on Earth — according to a recent Mexican study. At the same time, the number of dual citizens has risen dramatically in countries that collect data on this issue, on average doubling in the decade or so between censuses.