Japan's Supreme Court has upheld a law that effectively requires transgender people to be sterilized before they can have their gender changed on official documents.

The court said the law is constitutional because it was meant to reduce confusion in families and society. But it acknowledged that it restricts freedom and could become out of step with changing social values.

The 2004 law states that people wishing to register a gender change must have their original reproductive organs, including testes or ovaries, removed and have a body that "appears to have parts that resemble the genital organs" of the gender they want to register.

The unanimous decision by a four-judge panel, published Thursday, rejected an appeal by Takakito Usui, a transgender man who said forced sterilization violates the right to self-determination and is unconstitutional.

Usui, 45, had appealed to the top court after he unsuccessfully requested lower courts to grant him legal recognition as male without having his female reproductive glands surgically removed.

Despite the unanimous decision, presiding justice Mamoru Miura joined another justice in saying that while the law may not violate the constitution, "doubts are undeniably emerging," according to Usui's lawyer, Tomoyasu Oyama.

The two judges proposed regular reviews of the law and appropriate measures "from the viewpoint of respect for personality and individuality," according to Japanese media reports.

The Supreme Court decision ended Usui's legal battle, but he and his lawyer said the opinions in the ruling left them with hope.

"I think the ruling could lead to a next step," Usui told a news conference. "I hope to find what constitutes a family of my own that does not fit the traditional mold."