This article examines the visualization of “the mentally ill” (seishinbyōsha) in Japan by focusing on a psychiatric report on home confinement published in 1918. It argues that the authors of the report, psychiatrists Kure Shūzō and Kashida Gorō, mobilized a representational strategy developed mainly in a context of colonialism called the “documentary mode” to convince readers of the scientific nature of their photographs, drawings, and floor plans of home confinement. The documentary mode enabled the psychiatrists to present their viewpoint—that “the mentally ill” belonged to a distinct group deserving sympathy and medical care—as the most truthful claim, above those made by lawmakers, officials, and families. Considering the visual technologies that helped define “the mentally ill” shows how this category was in flux in the early twentieth century, subject to redefinition in the hands of those who claimed to identify, picture, and tally its members in the most truthful way.