Daul nationality is not technically against the Japanese law.
Please refer to the following English translation of the Nationality Law of Japan from the official website of the Ministry of Justice Japan.
http://www.moj.go.jp/ENGLISH/CIAB/law01.html First of all, note that the age you must choose your nationality is 22. This is according to Article 14 where it says,
"A Japanese national having a foreign nationality shall choose either of the nationalities before he or she reaches twenty two years of age if he or she has acquired both nationalities on and before the day when he or she reaches twenty years of age or, within two years after the day when he or she acquired the second nationality if he or she acquired such nationality after the day when he or she reached twenty years of age."
However, the law doesn't actually tell you to _surrender_ your other nationality. It just says you have to _keep trying_. Please refer to Article 16 for details where it says,
"A Japanese national who has made the declaration of choice shall endeavour to deprive himself or herself of the foreign nationality."
So bottom line, you won't get "caught" even if they find out you have dual nationality after 22 years of age. Just tell them, "I'm trying" and you'll be okay. If you don't believe me, phone the Ministry or your local Japanese Embassy yourself. Anonymously if you wish. I know lots of people who actually inquired about this, and they're still free. Now, spread the word.
The other thing though, is that there seems to be a trick on "how" you should use your two different passports.
I know a person who showed both passports and was
told by airport immigration officails themselves to only use Japanese passports when leaving and entering Japan even though you have another (legal) nationality. It is the same when entering the foreign country, i.e.: do
not show the Japanese passport, only the passport for the country you are entering.
If you are travelling from Japan to a third country, use the Japnese passport to leave and the passport you think will be the most easily accepted in the other country, the person says.
For some reason, showing two passports seems to put immigration into a tizzy and many questions will follow.