Reserved Japanese nationality
Article 12 Nationality Law
A Japanese citizen who acquired the nationality of a foreign country through birth and who was born abroad shall retroactively lose Japanese nationality to the time of birth unless he/she indicates an intention to reserve Japanese nationality pursuant to the provision of the Family Register Act (Act No. 224 of 1947).
Renounce Japanese nationality
(1) A Japanese citizen having foreign nationality may renounce Japanese nationality by notification to the Minister of Justice.
(2) The person making the notification provided for in the provisions set forth in the preceding paragraph shall lose Japanese nationality at the time of the notification.