Japanese law strictly adheres to the principle of ius sanguinis, whereby citizenship is attributed by parentage rather than place of birth. Japanese citizenship may also be acquired through naturalisation, with restrictions.
Under the law, a person who possesses a Japanese and foreign citizenship(s) must choose to renounce either the Japanese nationality or their foreign one(s) by his/her 22nd birthday, or within 2 years after the day of acquiring the second nationality if such a nationality was acquired after having reached the age of 20.
The law does not specify any penalties against dual nationals who do not pick a nationality. It instead states that the justice minister reserves the right to warn them to choose a nationality. If a dual national does not make a choice within a month of receiving the warning, their Japanese nationality is automatically revoked. The law also stipulates that those who naturalise to another country will automatically lose their Japanese citizenship.
Japanese individuals may come to possess dual nationality in a variety of scenarios. One such case is when a person is born of a mother having Japanese nationality and a father having a nationality of the foreign country which adopts the “principle of paternal blood” as the rule of nationality. Similarly, a person born of a father or a mother having Japanese nationality and another parent having a nationality of the foreign country which adopts the “principle of both-parental blood” would also acquire dual nationality. A person would also possess dual citizenship should he/she be born, of a father and/or of a mother having Japanese nationality, in a foreign country which adopts the principle of ius soli as the rule of nationality.
Other instances include, when a Japanese national has acquired a foreign nationality as the result of acknowledgement by a father of a foreign nationality, the adoption with a person of a foreign nationality, or the marriage to a person of a foreign nationality. There may also be cases of dual citizenship where a person still holds a foreign nationality after acquiring Japanese nationality by naturalisation. In all cases, dual citizenship is not allowed after the person becomes 22 years of age.