US & Japanese Passports

Dual citizenship is allowed to those who had it prior to January 1, 1985 regardless of age, contrary to JT's post there is no cutoff of age 20 at the time.

My sister and I are both dual citizens and were under the age of 20 when the law was passed. You are grandfathered if born before January 1, 1985. However, if you haven't kept your passport current, you will have to file additional paperwork to reobtain it.

My American born child will have to choose between the two by age 22 technically, but unless you either denounce or declare you can maintain the two, until requested by the Minister of Justice as the law states.

I had never heard of it being enforced except in the one case mentioned in the forum. Depending on when you were born, you may have the right to reclaim or reapply for your Japanese passport if it is taken away as mentioned. The case could also be made that an immigration official is not the Minister of Justice.

More of an official answer to which passport to use. See under the section Which Passport to Use at the US embassy's website. http://japan.usembassy.gov/e/acs/tacs-7118.html
It does confirm to use the US passport to leave and enter the US while suggesting to use whichever passport works best at the other end.