This paper explores how Japanese officials and others conceptualized police power at particular junctures in imperial Japanese history (1868–1945). It does so by synthesizing prior scholarship on the Japanese police into a broader genealogy of the police idea in prewar Japan, beginning with the first translations and explanations of police in the Meiji period, the changing perceptions of the police in the 1910s, and the evolution from the “national police” idea in the 1920s to the “emperor's police” in the late 1930s. The essay proposes that the police idea in Japan (and elsewhere) can be read as a boundary concept in which the changing conceptions of police power demarcate the shifting relationship between state and society. Indeed, it is the elusiveness of this boundary that allows for police power – and by extension, state power – to function within society and transform in response to social conditions. Approached in this way, the essay argues that the different permutations of the police idea index the evolving modality of state power in prewar Japan, and thus allows us to reconsider some of the defining questions of imperial Japanese history.