The appellant of final appeal, who was born to a father who is a Japanese citizen and a mother who has nationality of the Republic of the Philippines, a couple having no legal marital relationship, submitted a notification for acquisition of Japanese nationality to the Ministry of Justice in 2003 on the grounds that he/she was acknowledged by the father after birth, but the minister determined that the appellant had not acquired Japanese nationality due to the failure to meet the requirements for acquisition of Japanese nationality. In this case, the appellant sued the appellee, seeking a declaration that the appellant has Japanese nationality.

Court ruling

In addition, it seems that other states are moving toward scrapping discriminatory treatment by law against children born out of wedlock, and in fact, the International Covenant on Civil and Political Rights and the Convention on the Rights of the Child, which Japan has ratified, also contain such provisions to the effect that children shall not be subject to discrimination of any kind because of birth. Furthermore, after the provision of Article 3, para.1 of the Nationality Act was established, many states that had previously required legitimation for granting nationality to children born out of wedlock to fathers who are their citizens have revised their laws in order to grant nationality if, and without any other requirement, it is found that the father-child relationship with their citizens is established as a result of acknowledgement.

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In light of these changes in social and other circumstances at home and abroad, we should say that it is now difficult to find any reasonable relevance between the policy of maintaining legitimation as a requirement to be satisfied when acquiring Japanese nationality by making a notification after birth, and the aforementioned legislative purpose.
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Considering that acquisition of Japanese nationality means a lot to people in order to enjoy the guarantee of fundamental human rights and other benefits in Japan, we should say that the disadvantages that children would suffer from the above-mentioned discriminatory treatment cannot be overlooked, and we must say that we can hardly find reasonable relevance between such discriminatory treatment and the aforementioned legislative purpose. In particular, between children acknowledged by Japanese fathers before birth and those acknowledged after birth, it is difficult to find a difference in general in terms of the level of the tie with Japanese society developed through their family life with Japanese fathers, and it is also difficult to explain the reasonableness of the policy of applying the above-mentioned distinction when granting Japanese nationality from the perspective of the level of the tie with Japanese society. In addition, under the Nationality Act that adopts the principle of jus sanguinis, if, despite the fact that children born out of wedlock to Japanese mothers can acquire Japanese nationality by birth, children born out of wedlock who satisfy only the requirement of being acknowledged by Japanese fathers after birth are not allowed to acquire Japanese nationality even by making a notification, we should say that such a situation is somewhat inconsistent with the basic stance of the Act from the perspective of gender equality.
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For the reasons stated above, we should conclude that although the legislative purpose itself from which the Distinction is derived has a reasonable basis, reasonable relevance between the Distinction and the legislative purpose no longer exists due to the changes in social and other circumstances at home and abroad, and today, the provision of Article 3, para.1 of the Nationality Act imposes an unreasonable and excessive requirement for acquiring Japanese nationality. Moreover, since the Distinction involves another distinction described in (2)(d) above, we must say that it causes a child born out of wedlock who satisfies only the requirement of being acknowledged by a Japanese father after birth to suffer considerably disadvantageous discriminatory treatment in acquiring Japanese nationality, and even if we take into consideration the discretionary power vested in the legislative body when specifying requirements for acquisition of Japanese nationality, we can no longer find any reasonable relevance between the consequence arising from the Distinction and the aforementioned legislative purpose.
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Therefore, we must conclude that at the time mentioned above, the Distinction amounted to unreasonable discrimination, and the provision of Article 3, para.1 of the Nationality Act was in violation of Article 14, para.1 of the Constitution in that the provision caused the Distinction.

https://www.courts.go.jp/app/hanrei_en/detail?id=955