As I do not have Japanese FEP so please let me excuse in English.
If you obtain Canadian citizenship on naturalization, you can’t hold Japanese because of Article 11 of Japanese Nationality Laws.
If you got your Canadian citizenship through birth in Canada, or in any country from one or both of your parents, you can keep, as Canadian Government does not approve the renunciation of Canadian citizenship for choosing only Japanese, despite Japanese Nationality Laws Article 14 enforces the choice.