The idea of equality appeared in Japan only after the country had entered into modern times and was embodied at the legal level in the abolishment of the shinoukousho system during the Meiji era. Before the Second World War，however，the hierarchical differences of social status were still very common and the idea of equality had not yet entered into the mind of the majority of Japanese people. This situation was greatly changed by the 1946 Constitution of Japan，which provided for the principle that all citizens are equal before the law and established an American-style judicial review system. Based on these constitutional provisions，Japanese courts at various levels made a series of decisions on the issue of equal protection in the fields of marriage，family，and voting rights，at the same time，carried out discussions on the meaning and criteria of “reasonable differences”，thereby upholding and further developing the constitutional principle of equal protection.