## Facts

- A statutory proviso in Japan’s Civil Code set a child born out of wedlock’s intestate inheritance share at one-half the share of a child born in wedlock.
- A died intestate in July 2001, leaving both children born in wedlock (Y) and children born out of wedlock (X) as heirs.
- Y sought judicial division of the estate applying the half-share rule to X.
- X challenged the proviso as violating the constitutional guarantee of equality under the law (Article 14(1)).
- The family court applied the half-share rule; the Tokyo High Court affirmed, reasoning the rule was not unconstitutional as of the inheritance’s commencement (July 2001), relying on prior Supreme Court precedent.

## Issues

1. Whether the Civil Code proviso assigning children born out of wedlock one-half the inheritance share of children born in wedlock violates the constitutional equality guarantee (Article 14(1)).
2. If unconstitutional, from what point in time the unconstitutionality applies and how the ruling affects other inheritances and previously settled estate divisions.

## Decision

- The Grand Bench held the half-share proviso violated the constitutional equality guarantee since July 2001 at the latest.
- The Court quashed the Tokyo High Court judgment and remanded for recalculation and division without applying the unconstitutional proviso.
- The Court limited broader effects by stating the ruling would not disturb legal relationships fixed by final adjudications or estate-division agreements made in reliance on the proviso for other inheritances commencing between July 2001 and the date of the decision.

## Legal Principles

- Equality under the law prohibits discriminatory legal treatment absent a reasonable basis consistent with the nature of the matter regulated.
- Legislative discretion in designing inheritance and family law is constrained; discrimination becomes unconstitutional when its justification no longer reasonably supports the differential treatment under prevailing social conditions.
- Constitutional validity can change over time as social conditions, family forms, and legal and international norms develop.
- When declaring a rule unconstitutional, a court may limit the ruling’s disruptive effect to protect the finality of judgments and the stability of settled property and family relations.

## Conclusion

The court held that denying equal intestate inheritance shares to children born out of wedlock violated constitutional equality by July 2001 at the latest, ordered recalculation of the parties’ shares without the discriminatory proviso, and preserved stability by insulating finalized judgments and agreements in other estates from being reopened.


X v. Y, 67 Minshū 1320 (Sup. Ct. Japan 2013)