In this paper, I intend to present a genealogy as to how crimes by mentally disordered people came to be recognized as a problem to be dealt with by special legal treatments, i. e. security measures. Though the security measures have never been realized in Japan, their introduction have been argued as early as the Taisho era. Considering the history of this argument, “mentally disordered criminals” were not considered a problem because they were dangerous. Rather, it was felt that they were made “dangerous and irresponsible” subjects due to the debate concerning the security measures themselves.