Acquisition of Nationality by Acknowledged Children
For example, if you can obtain the judgment by the DNA Test that the father is Japanese, and then your daughter or son can be Japanese when you applied.
Article 3 Nationality Law
(1) In cases where a child acknowledged by the father or mother is under twenty years of age (excluding a child who was once a Japanese citizen) and the acknowledging father or mother was a Japanese citizen at the time of the birth of the child, Japanese nationality may be acquired through notification to the Minister of Justice if that father or mother is currently a Japanese citizen or was so at the time of death.
(2) The person making notification provided for in the provision set forth in the preceding paragraph shall acquire Japanese nationality at the time of the notification.