The issue before the Supreme Court was whether a part of a provision in the Japanese Nationality Act conformed with Article 14.1 of the Japanese Constitution, which prohibits discrimination based on race, belief, sex, social status, or lineage. The provision at issue did not grant Japanese nationality to a child born out of wedlock to a non-Japanese mother and a Japanese father––even if the father formally declares and recognizes the father-child relationship––unless the child obtains legal recognition as a child of the man and the woman through their marriage. The Supreme Court first noted that the Japanese Nationality Act does not grant Japanese nationality to a child in the aforementioned situation although it recognizes a parent-child relationship and grants Japanese nationality to a child born out of wedlock if (1) the child’s mother was Japanese or (2) the child’s Japanese father filed for the recognition of the father-child relationship before the child’s birth. The Court found that, while creating this distinction was reasonable at the time of the legislation, such a distinction amounted to unjustifiable discrimination in present day Japan. Thus, the Supreme Court found that the part of the provision at issue was unconstitutional and invalid. In its reasoning, the majority opinion stated, inter alia, “under the Japanese Nationality Act that adopts the principle of jus sanguinis, maintaining a distinction in terms of eligibility to have Japanese nationality based on whether the Japanese parent is the mother or the father of the child at issue does not accord with the basic principle of equality of the sexes.”
本件の争点は、当時の日本の国籍法の一部の合憲性である。当時の国籍法は日本国民である父の非嫡出子について、父母の婚姻により嫡出子たる身分を取得した者に限り日本国籍の取得を認めている。同じく日本国民である父から認知された子でありながら父母が法律上の婚姻をしていない非嫡出子は、その余の同項所定の要件を満たしても日本国籍を取 得することができないという区別が生じていた。最高裁は、このような区別を設けることは、立法当時は合理的であったが、現在の日本では合理的理由のない差別であると判断し、問題となった規定の一部を違憲・無効とした。その理由として、「日本国民である母の非嫡出子が出生により日本国籍を取得するにもかかわらず，日本国民である父から出生後に認知されたにとどまる非嫡出子が届出による日本国籍の取得すら認められないことには，両性の平等という観点からみてその基本的立場に沿わない」と述べた。