This chapter studies how an amorphous group of population experts became prominent in policymaking during the 1920s, when the phrase “population problem” entered the Japanese lexicon. This catchall term was used to refer to various kinds of socioeconomic ills, all of which were deemed to require state intervention. The chapter first describes how policy-oriented debate about the “population problem” developed in the 1920s, mostly among social scientists familiar with the “Karl Marx versus Thomas Malthus” argument initially introduced from the west. It then explores how the “population problem” became a policy priority in the late 1920s, by examining research and policy discussions that took place in the Investigative Commission for Population and Food Problem. By scrutinizing policy deliberations within the Investigative Commission about emigration and population control, the chapter shows that population experts were necessarily always in line with the government’s agenda. It also points out that the policy deliberation and research mobilized by the Investigative Commission laid a critical foundation for the institutionalization of government research on population problems, and for the establishment of the Ministry of Health and Welfare, both of which were realized as Japan entered war with China in 1937.