A person born to a Japanese father or mother is Japanese at birth. A person born in Japan whose parents are both unknown or without nationality is also Japanese. Birth in Japan alone to identified foreign parents is not general jus soli.
Sources: Japan Nationality Act (Japanese Law Translation) · Japan Ministry of Justice — Nationality Q&A
A spouse of a Japanese may naturalize after three years of residence (or one year if married three or more years), instead of five.
A child is a Japanese citizen if the father or mother is a Japanese citizen at the time of birth, or if the father died before birth while a Japanese citizen. Acknowledgment after birth by a Japanese father can support nationality acquisition by notification under Article 3 when statutory age and status conditions are met.
Sources: Japan Nationality Act (Japanese Law Translation) · Japan Ministry of Justice — Nationality Q&A
A foreign national may acquire Japanese nationality by naturalization with permission of the Minister of Justice. Ordinary minimum conditions include five or more years continuous domicile in Japan, majority and capacity, good conduct, livelihood, and renunciation or loss of prior nationality, plus constitutional-compliance conditions. Meeting the statutory minima does not create an entitlement to permission.
Sources: Japan Nationality Act (Japanese Law Translation) · Japan Ministry of Justice — Nationality Q&A