Extracted from Wikipedia (
http://en.wikipedia.org/wiki/Japanese_nationality_law):
Japan is a jus sanguinis state, meaning that it attributes citizenship by blood, not by location of birth. Article 2 of the Nationality Act provides three situations in which a person can become a Japanese national at birth:
1. When either parent is a Japanese national at the time of birth
2. When the father dies before the birth and is a Japanese national at the time of death
3. When the person is born on Japanese soil and both parents are unknown or stateless
A system for acquiring nationality after birth is also available. If an unmarried Japanese father and non-Japanese mother have a child, and the parents later marry and the Japanese father acknowledges paternity, the child can acquire Japanese citizenship, so long as the child has not reached the age of 20. Japanese nationality law effective from 1985 has been that if the parents are not married at the time of birth and the father has not acknowledged paternity while the child was still in the womb, the child will not acquire Japanese nationality.[1][2] However, Japan's Supreme Court ruled in 2008 that denying nationality to children born out of wedlock to foreign mothers is unconstitutional