日本的平等观念出现于近代以后，明治政府“四民平等”政策的推行是这一观念在法律层面的体现，但在第二次世界大战前，等级差别仍非常普遍，“人人平等”这一思想本身还没有进入大部分人的观念之中。1946年制定的《日本国宪法》使这一状况有了很大改观，新宪法明确规定了国民在法律面前的平等并确立了美国式的司法审查制度。日本各级法院以该规定为基础，针对婚姻家庭领域和选举权等平等保护问题作出了一系列判决，同时针对“合理差别”的含义和审查标准进行了讨论，维护了《日本国宪法》的平等保护原则，同时发展了平等保护理论。
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.
