The author spent months at the Eastern Police Station, a large station in Sapporo City, observing detectives in operation. The study included observation and the administration of questionnaires at the Hokkaido Police Academy, the police station, and the mobile squad attached to the headquarters. An outline of the formal legal system of criminal investigation covers voluntary investigation, arrest, detention at the police station, interrogations and confessions, and the admissibility of evidence. A review of the natural sequence of criminal investigation in Japan considers investigation by patrol officers, investigation of theft crimes, the investigation of forcible crimes, and the investigation of organized crimes. A description of investigative activity considers how detectives think and behave in various aspects of the criminal investigation followed by an analysis of why police investigators think and behave as they do. The study portrays hurried detectives under pressure to clear ever more crimes, torn between following the rules and evading them; a system of detention and questioning that makes it almost impossible for suspects to maintain their innocence; and simultaneously, detectives dedicated to doing the job right despite all obstacles. A 68-item bibliography and appended supplementary information