PROBLEM TO BE SOLVED: To provide a device for measuring a pupil diameter of a subject who relaxed.SOLUTION: The device for measuring a pupil diameter includes: a half mirror 21 placed in front of at least one of the right eyeball or the left eyeball while fixing the position of the eyeball of the subject an imaging part 2 for capturing the eyeball from light incident through the half mirror 21 a control part 1 for extracting a pupil area from the captured eyeball image, and calculating a pupil diameter from the extracted pupil area and a display 3 for presenting the value of the calculated pupil diameter. A forehead rest, a chinrest, the half mirror 21, and the imaging part 2 are arranged so as not to block light L incident from above the subject to the eyeball while fixing the position of the eyeball of the subject.
