Japanese nationality law

Japanese nationality is a legal designation and set of rights granted to those people who have met the criteria for citizenship by parentage or by naturalisation. Nationality is in the jurisdiction of the Minister of Justice and is generally governed by the Nationality Law of 1950.
History.
Prior to 1947, in an example of jus matrimonii, marrying a Japanese citizen would enter the foreign spouse into the family registry of said citizen, making them a citizen as well (or for the Japanese spouse to lose their family registry, and by extension their Japanese citizenship). Yakumo Koizumi, the first-ever naturalised Japanese subject, gained Japanese citizenship in such a manner.
Nationality by birth.
Japan is a strict "jus sanguinis" state as opposed to "jus soli" state, meaning that it attributes citizenship by blood and not by location of birth. In practice, it can be by parentage and 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, the parents later marry, and the Japanese father acknowledges paternity, the child can acquire Japanese nationality, 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.
Following this, the conservative Liberal Democratic Party and others, claiming the possibility of false recognition of paternity, suggested mandatory DNA testing. This was rejected by the Democratic Party, and instead a bill was passed in 2009 allowing photographs of the father and child and scientific testing to be requested as evidence in cases of doubt.
Naturalization.
Naturalization in Japan requires the applicant to give up their current citizenship(s) either before or shortly after, depending on the nationality, the naturalization takes place if the loss of nationality does not occur automatically. Although there are rules in place, Japanese government does not strictly impose rules for the naturalization process, as the exact process for each specific nationality depends on Japan's international relations and agreements with the given country. Basic naturalization requirements also differ from person to person regardless of their nationality and depending on the applicant's current status in Japan. Unlike most other countries, the applicant does not have to have been a permanent resident to be eligible to apply for Japanese naturalization.
The outlined 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.
Those that naturalize must choose a legal name, like other Japanese, that consists of all or any mix of Japanese hiragana, katakana, and approved kanji (for example, Adrian Havill became Eido Inoue, 井上エイド). Sometimes applicants were given advice on Japanese names, but choosing a Japanese sounding/appearing name was never a requirement; there are examples through history of naturalized Japanese choosing legal names that did not appear ethnically Japanese. However, in 1983, the Ministry of Justice revised its manuals and application guides and examples to make it clear that using names of non-Japanese origin can be acceptable. They must also choose a registered domicile.
The application has to be made in person to the Ministry of Justice branch office with responsibility for the city where the applicant lives. A booklet will be given to the applicant at the first visit which explains every needed document and processes explained in Japanese.
The naturalization process has three stages.
The booklet given at the beginning of the process lists every document that the applicant needs from their home country and their country's embassy in Japan. The applicant needs to be able to speak and express himself/herself in Japanese and be able to answer the interview questions in Japanese. The interviewer will ask questions about the form applicant filled and about why applicant wants to acquire Japanese citizenship. At the end, there may be a written test at an elementary school second grade level.
After the documents are sent to Tokyo for processing at the Ministry of Justice headquarters, it can take from 8 to 10 months (or longer depending on the applicant) from the first application. The applicant will be called by their interviewer about the decision once the interviewer gets the results from Tokyo.
Loss of citizenship.
Loss of citizenship requires the approval of the Minister of Justice.
A Japanese national is assumed to have renounced their nationality upon naturalization in any foreign country, or if they otherwise acquire foreign nationality by their own choice. This is not triggered by acquisition of foreign nationality that is not by the individual's own choice, such as acquisition of foreign nationality at birth, or automatic acquisition of US citizenship due to adoption or a parent's naturalization.
Under the revisions made to the Nationality Law in 1985, Articles 14 and 15 require any person who holds multiple citizenship to make a "declaration of choice" between the ages of 20 and 22, in which they choose to renounce either their Japanese nationality or their foreign citizenship(s). Failure to do so entitles the Minister of Justice to demand a declaration of choice at any time. If the required declaration is not made within one month, their Japanese nationality is automatically revoked. A renunciation of foreign citizenship made before Japanese officials may be considered by a foreign state as having no legal effect as is the case with, for example, United States citizenship.
Japanese nationals who hold multiple citizenship by birth, and who do not wish to lose their Japanese citizenship, are required to declare their desire to retain Japanese citizenship by the age of 21. Part of fulfilling this requirement is to "make an effort" to renounce other citizenships once they have declared their intent to retain Japanese nationality. This may be difficult for some Japanese with foreign nationality, for example, Iranian nationals cannot renounce their Iranian nationality until age 25.
A Japanese national does not lose their nationality in situations where citizenship is acquired involuntarily such as when a Japanese national marries an Iranian national. In this case they automatically acquire Iranian citizenship and are permitted to be an Iranian-Japanese dual national, since the acquisition of the Iranian citizenship was involuntary.
Though it is unknown whether it has ever happened, citizenship can also be lost if a person becomes a civil servant of a foreign government, should their role be felt to contradict what it means to be a citizen of Japan.
In November 2008, Liberal Democratic Party member Tarō Kōno submitted a proposal to allow offspring of mixed-nationality couples in which one parent is Japanese to have more than one nationality. The proposal also calls for foreigners to be allowed to obtain Japanese nationality without losing their original citizenship.
Dual nationality.
Dual citizenship of Japan and another country is prohibited in some cases due to the provisions for loss of Japanese nationality when a Japanese national naturalizes in another country (see "Loss of citizenship" above), and the requirement to renounce one's existing citizenships when naturalizing in Japan (see "Naturalization" above). There are still some ways in which a person may have dual citizenship of Japan and another country, including:
Travel freedom.
In 2019, Japanese citizens had visa-free or visa on arrival access to 189 countries and territories, ranking the Japanese passport as tied for first (along with Singapore) in the world according to the Henley Passport Index.
In 2017, the Japanese nationality is ranked twenty-ninth in the Nationality Index (QNI). This index differs from the Visa Restrictions Index, which focuses on external factors including travel freedom. The QNI considers, in addition to travel freedom, on internal factors such as peace & stability, economic strength, and human development as well.