Six local governments in Japan now recognise same-sex partnerships and many big companies, including insurers and hospitals, treat gay couples the same way as heterosexual spouses.
But activists, lawyers, and lawmakers are now calling for the legalisation of same-sex marriage.
"Among the Group of Seven industrialized nations, only Japan has not yet introduced a same-sex marriage or same-sex partnership system at the state level," Ken Suzuki, a law professor at Meiji University, told Kyodo news agency.
"It is a shared awareness among advanced nations that excluding same-sex couples from the legal marriage framework constitutes discrimination against lesbians and gays."
Assembly member Aya Kamikawa told Kyodo said that national same-sex marriage legislation often followed moves made locally.
"The development in six municipalities in Japan is expected to advance to the next stage at the state level," said Kamikawa, a transgender woman. "The legal system, including marriage, should be equally open to everyone, regardless of sexual orientation or gender identity. Everyone should be guaranteed the equal right of choice, otherwise it would be discriminatory."
A recent symposium in Tokyo, organised by the Japan Federation of Bar Associations to debate the legalising same-sex marriage, was attended by 100 people and broadcast in several cities.
Takeshi Shiraishi, a public school teacher in Tokyo who has lived with his male partner for 25 years, told the symposium he and his partner were not considered family and were charged double when renting an apartment in the past.
"We hope we can continue living together in peace...," he told the symposium. "In that case, we expect the remaining survivor to legally maintain what we have generated together...It is an issue regarding the constitutionally guaranteed equality before the law."