Dual Nationality Question

Hey guys, I’m 17 and half Japanese and American and currently reside in Japan. I’m concerned about the citizenship laws because apparently I have to choose between keeping only my Japanese or American citizenship once I turn 18. I prefer to keep my American citizenship, so will I have to permanently denounce my Japanese one? Does this mean I will no longer be a Japanese citizen once 18, and will I no longer be able to have two passports even though I am Japanese and American? Thanks.

by dwen_