This paper is an effort to situate interlinguistics and Esperanto studies in the social context of modern Japan. The origin of interlinguistic ideas in Japan was distinct from developments in Europe, in that English functioned as the bridge language to learn Western civilization from the very beginning of Japan’s modernization, while it was the lack of a suitable regional lingua franca that motivated the Europeans to search for a planned language. After the examination of some Japanese pioneers in interlinguistics, the main focus will be upon diverse traditions of Esperanto studies in Japan. These include the endogenous (inward-looking) tradition, socially engaged interlinguistics, the post-war ambivalence of the Esperanto movement toward scientific theorizing, and the gradual rise of macro-sociolinguistic approaches from the 1990s.