To provide a device which can diagnose a brain disease by observing a pupil diameter. A device for diagnosing a brain disease, equipped with a display image control section (3), an imaging section (5) and a judgment section (7), wherein the display image control section (3) can vary a display image displayed on a display section on the basis of a first pattern, the imaging section (5) can take an image of the pupil of a user who watches the display image, and the judgment section (7) can determine whether or not the user is affected with a brain disease using a pattern of the time-dependent change in the size of the pupil of the user which is imaged by the imaging section and the first pattern.