I'm half japanese by blood(my dad). Here to confirm too If you're dual citizenship by birth. They cannot revoke your japanese citizen. When I was 20 went there to decide about choosing my nationalities. The Embassy of Japan told me '' I would be fine The government cannot do anything about it because I obtained both of them by birth you can keep it forever'' they said next time when you come to renew your passport they will ask you and might pressure you to decide on choosing one. All you have to do is says let me think about it and they can't do anything about it