Japanese nationality is generally governed by the Nationality Law of 1950.
Japan is a jus sanguinis state as opposed to Jus soli state, meaning that it attributes citizenship by blood but not by location of birth. However, in practice, it is by parentage but not by descent. Article 2 of the Nationality Act provides three situations in which a person can become a Japanese national at birth:
A system for acquiring nationality by birth 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. However, Japan's Supreme Court ruled in 2008 that denying nationality to children born out of wedlock to foreign mothers is unconstitutional.
Japan does not allow dual citizenship and anyone who willingly applies and obtains foreign citizenship automatically forfeits Japanese citizenship. The exception is where one is given special honourably citizenship by foreign state. In this instance, one has not applied to acquire foreign citizenship so one will not forfeit Japanese citizenship by retaining foreign citizenship.
Japanese who acquire dual citizenship by default for the reason such as birth (due to one of their parent being non Japanese) or by location (when born in Jus soli state) can retain their dual citizenship until age 20, at which point, failure to renounce foreign citizenship will result in automatic loss of Japanese citizenship. Children and descendant of Japanese migrant often are not Japanese citizens for this reason.
Because of Japanese prohibition of dual nationality, people of Japanese descent who acquire birthright foreign citizenships cannot maintain Japanese citizenship without renouncing their foreign citizenship at the age of majority (20 years). Sansei (third generation Japanese in foreign countries) commonly cannot claim Japanese citizen as their nisei (second generation) parent will have lost Japanese citizenship prior to sansei's birth. However, people of Japanese descent can get residency and work permit based on their ancestry, allowing them to initiate the naturalisation process by migrating to Japan.
The Minister of Justice must approve all applications for naturalization. Review of an application generally takes about one year.
The criteria for naturalization are provided in Article 5 of the Nationality Act:
The Minister of Justice may waive the age and residence requirements if the applicant has a special relationship to Japan (for example, a Japanese parent).
The Nationality Act also provides that the Diet of Japan may confer Japanese nationality by special resolution to a person who has provided extraordinary service to Japan. However, this provision has never been invoked.
For many years naturalized citizens were required to adopt a Japanese family name. This requirement was abolished in the late 1980s. A well-known example of someone who did not adopt a Japanese name is Masayoshi Son, the wealthiest man in Japan as of 2007, who naturalized using his Korean family name rather than the Japanese family name he used during his youth.
Loss of citizenship also requires the approval of the Minister of Justice.
A Japanese national is assumed to have renounced citizenship upon naturalization in any foreign country, although a formal report (on a form available at embassies overseas) from the renouncing person is generally required to finalize this process.
After the Nationality Law was revised in 1985, articles 14 and 15 require any person who holds multiple citizenship to make a declaration of choice between the ages of twenty and twenty-two, in which they choose to renounce either their Japanese citizenship or their foreign citizenship(s). If they fail to do so, the Minister of Justice may demand a declaration of choice at any time, and if the citizen fails to make the declaration within one month, their Japanese citizenship is automatically revoked. A renunciation of foreign citizenship made before Japanese officials may be considered by the foreign state as having no legal effect. This is the case with, for example, United States citizenship.
Japanese citizens who hold multiple citizenship by birth will be required to declare that they want to retain their Japanese citizenship by the age of 21. They also have to "make an effort" to renounce other citizenships once they have declared to retain their Japanese citizenship. This may be difficult for some Japanese with foreign nationality. For example, Iranian nationals cannot renounce their Iranian nationality until age 25, so Iranian-Japanese dual nationals who were born to an Iranian father will have to renounce their Japanese nationality. Actually, even this, since the citizenship was acquired involuntarily, it is not necessary for them to renounce their Iranian nationality. However, exercising their other citizenship in Japan is an expatriating act. For example, for the sake of partaking in the JET Program, one goes to Japan as a Canadian citizen, then, returns to Japan as a Japanese. Another example is a Japanese citizen trying to get a job as an Assistant Language Teacher, a position which is not open to Japanese nationals, if a Japanese gets a visa to work on their Canadian or British passport, this too becomes expatriating act. So if a child is born with dual nationality or acquires it as a child as a result of the parents naturalizing, the child may hold dual nationality, but never allowed to exercise his or her rights as a foreigner in Japan, or they will be committing an expatriating act. This is an advantage, in particular, to Japanese who acquired a citizenship they may be unable to renounce (as in the case of Japanese citizens, born in Japan, to a father born in an Arab country or Israel).
A Japanese does not lose his or her citizenship if another citizenship is acquired involuntarily. If a Japanese woman marries an Iranian man, she will automatically acquire his citizenship. She will be allowed to be an Iranian-Japanese dual national, since the acquisition of the Iranian citizenship was involuntary.
On November 14, 2008 The Japan Times reported that Liberal Democratic Party member Taro Kono had submitted a proposal to allow offspring of mixed national couples, one of whom being Japanese, to have more than one nationality. The proposal also calls for foreigners to obtain Japanese citizenship without giving up their original one.