The basic philosophy of eugenics is owing to an idea classifying human beings drastically in two groups, superiority and inferiority, by the name of "science." Such concept has influenced greatly on many fields and been accepted unconsciously in Japan. However, recently many people are taking part in movements in order to get rid of such concept. Since establishment of "Kokumin Yusei Ho" was in the time of such discriminative concept being generally accepted, the author tries to examine historical background of the act particulary between 1935 and 1939. For that purpose policies of the government and the national assembly as well as basic ideas of research institutions, psychiatrists, and social workers are discussed in this paper hoping to help recent movements.