Around a century ago, in his “The Ideals of the East”, Okakura Tenshin (Kakuzô) proclaimed that “Asia is one”. This phrase, quoted repeatedly ever since, has been interpreted as representative of the ideology of Pan-Asianism (Han-Ajiashugi) or Asianism (Ajiashugi) in Japan. However, Okakura's writings were not widely read in Japan during the Meiji era and his originally English writings were translated into Japanese only in the 1930s. It must have been other authors that defined Pan-Asianism as a comprehensive ideology and brought this ideology closer to politics, a sphere where pan-Asian approaches were mostly rejected until the 1910s. This paper introduces the writings of Kodera Kenkichi (1877–1949), a politician and long-time member of the Lower House of the Imperial Japanese Diet, and identifies his “Treatise on Greater Asianism” (1916) as a central work in the history of the ideology of Asianism in modern Japan.