There are two ways to lose Japanese citizenship: 1) As an adult, consciously acquire another citizenship. 2) If you acquired another citizenship involuntarily

There are two ways to lose Japanese citizenship: 1) As an adult, consciously acquire another citizenship. 2) If you acquired another citizenship involuntarily as a child (eg. being born), at 21 you are required to choose one nationality. If you do nothing, you will stay Japanese on paper.