This dissertation aims to answer comprehensively the simple, yet significant question of why and how population became a political problem in interwar Japan (late 1910s - late 1930s). During Japan’s interwar years, there was a growing call among social scientists, social reformers, and government elites to solve “population problem (jinkō mondai).” These Japanese intellectuals attributed the population problem in Mainland Japan (naichi) to a wide array of social ills including poverty, unemployment, and physical, mental, and moral degeneration, and considered various solutions to reform the Japanese population. The prevalence of this population discourse must be understood as an obvious symptom of the growing attention among contemporary Japanese intellectuals and bureaucrats to the population: the size and quality of the population became an object of knowledge and an objective of government. Moreover, the ambiguous, yet productive category of the Japanese population provides a revealing look at the complex social relations and colonial mobility in the Japanese Empire. This dissertation focuses on modern governmentality and imperialism that were embedded in the interwar discourse of the population problem. Using Michel Foucault’s conceptualization of discourse, I consider the population discourse to encompass different, or even conflicting agendas, languages, and movements that shaped and reshaped the population problem. The close reading of the arguments of different population discourses, including Neo-Malthusianism, the proletarian birth control and eugenics movement, feminist advocacy for voluntary motherhood, and the government's investigation into population problems, reveals the distinctive nature of Japan's interwar period in two senses: 1) a dynamic space where various discourses on population issues—particularly, birth control, eugenics, and population policy—continuously interwove sexual and biological issues with politico-economic ones; and 2) a crucial stage for reconstructing Japanese modernity through integrating scientific progressivism, social reformism, and imperial nationalism. In sum, in revisiting interwar Japan through the frames of governmentality and imperialism, my dissertation illuminates how the multiple discourses on population constituted and categorized desirable bodies to reproduce, and how these discourses intersected with modern subjectivities—namely, gender, nation, and class.