The ruling by the Nagoya District Court was the second to find a ban against same-sex marriage unconstitutional, out of four cases on the issue over the past two years. Japan's constitution says marriage is between a man and a woman.

"This ruling has rescued us from the hurt of last year's ruling that said there was nothing wrong with the ban, and the hurt of what the government keeps saying," lead lawyer Yoko Mizushima told journalists and supporters outside the court.

She was referring to a ruling in Osaka last year that the ban was not out of line with the constitution.

A Tokyo court later upheld the ban on same-sex marriage but said a lack of legal protection for same-sex families violated their human rights.

Tuesday's ruling was greeted with cheers from the activists and supporters waving rainbow flags outside the court.

Though opinion polls show some 70 percent of the public supports same-sex marriage, the conservative ruling Liberal Democratic Party (LDP) of Prime Minister Fumio Kishida opposes it.

Kishida in February sacked an aide who sparked outrage by saying people would flee Japan if same-sex marriage was allowed, but the premier remains noncommittal about it and has said discussions must proceed "carefully".

Nevertheless, more than 300 Japanese municipalities covering some 65 percent of the population allow same-sex couples to enter partnership agreements.

But the right is limited in scope. Partners cannot inherit each other's assets or have parental rights to each other's children. Hospital visits are not guaranteed.

Mizushima said the court in its ruling had noted that such partnership agreements were not fully sufficient, which she took as an encouraging sign.

The government pledged to pass a law promoting "understanding" of LGBT people before hosting the G7 summit, but opposition from conservatives delayed the law so much a watered-down version was only submitted to parliament the day before the summit began.

The initial draft stipulated discrimination on the basis of sexual orientation and gender identity should "not be tolerated" but was changed to "there should be no unfair discrimination", wording that critics say tacitly allows bigotry.

Japan has come under increasing pressure to change, both from other G7 members but also from economic lobbies, with businesses arguing that greater diversity is needed for international competitiveness.

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