Posted: Tue Jun 13, 2006 5:22 am-
You can use either passport.
I know someone who is Australian and British and use the Australian passport when he goes to Australia and the British passport when he comes back in the UK.the only thing you cannot do is check in with one passport and switch you passport when you arrive in the country.
Be careful though because if you are using your Japanese passport because as an example: if they have mandatory military service (I don't know if they have in Japan) they can keep you there and make you do it .or any other things like this.make sure you know what the laws (and what is mandatory for the citizens).also if you enter with you Japanese passport you will have no help from the Canadian embassy (because you are then considered a Japanese and they cannot help you) in case of trouble with the law or other type of help.
please read this:
http://www.cic.gc.ca/english/citizen/dualci_e.html