With regards to  the  estate of  P,  who died in  July 2001, the  appellees who are  P's  children born in  wedlock filed a petition for  a ruling on  the division of  P's  estate against the  appellants, who are P's children born out of  wedlock. Relying on  the  1995 Decision, the  court of  prior instance, the Tokyo High Court, determined that the  Provision was not  in  violation of Article 14, paragraph (1) of  the  Constitution, and concluded that P's  estate should be  divided based on  the  respective statutory shares in  inheritance of  the  appellees and  the  appellants as  calculated by  applying the  Provision. The appellants argued that the  Provision is   in  violation of  Article 14, paragraph (1) of   the  Constitution and therefore void.

Court ruling

The SCJ [Suprene Court of Japan] (GB [Grand Bench]) quashed the decision of the Tokyo High Court by concluding that the Provision was in  violation of  Article 14 paragraph (1) of the Constitution as of July 2001 at the latest.

The 2013 Decision emphasized that the  matters to be considered (such as tradition, social conditions and  public sentiments) change with time. Therefore, the reasonableness of  the rules should be subject to  constant examination and scrutiny in light of  the Constitution, which provides individual dignity and  equality under the  law.

Quotations from the court decision:

Article 14, paragraph (1) of  the  Constitution provides for  equality under the  law, and  this  provision should be  interpreted as  prohibiting  any discriminatory treatment by  law unless such treatment is based on  reasonable grounds in  relation to  the  nature of  the  matter. This is  the  case law established by  the  precedent rulings of  this court.
                                    xxx                                  xxx                                    xxxx
Even if   the  legal marriage system itself is  entrenched in Japan, it  is  now impermissible, as  a result of  such change in  the  recognition, to  cause prejudice to  children by reason of  the  fact  that their mother and father were not  in  a legal marriage when they were born - a  matter that the  children themselves had  no choice or  chance to correct. Rather, it can be said  that a notion that all  children must be given respect as  individuals and  that their rights must be protected has been established.

Taken from Akiko, Ejima, "Emerging Transjudicial Dialogue on Human Rights in Japan- Does It Contribute to the Production of a Hybrid of National and International Human Rights?," Departmental Bulletin Paper, Meiji University
https://m-repo.lib.meiji.ac.jp/dspace/bitstream/10291/17048/1/houkadaigakuinronshu_14_139.pdf