Japan is increasingly isolated among advanced economies in not having made provision for same-sex marriage or same-sex civil unions at the national level. The main political obstacle to legal recognition of same-sex couples is the Liberal Democratic Party (LDP), Japan’s dominant party, which has ruled the country with only two interruptions since 1955; many other parties support legalizing same-sex marriage, while the LDP does not. The literature suggests that the LDP secured its dominant position due to opposition fragmentation and ineffectiveness, and its own clientelist practices. Against this backdrop, cases are progressing through the Japanese judicial system seeking a finding that same-sex marriage is required under the 1947 Constitution. So far, the results of these cases have been mixed, with one case finding the current marriage law unconstitutional and two finding it constitutional.

This article contends that Japanese courts should find in favour of these claims and hold that same-sex marriage is constitutionally required. That is so not merely because it is the better position under existing legal doctrine relating to equality and non-discrimination under Article 14(1) of the Constitution, but also because the dominant party status of the LDP means that there is less reason in constitutional theory for courts to show deference to the legislature. In a dominant party system, there is less reason to expect that the government will pay sufficient heed to rights, while ‘counter-majoritarian’ difficulties with judicial review are less credible given the more ambiguous nature of the democratic mandate of the legislature in a dominant party system. This undercuts a significant justification given for judicial deference.