A mental fatigue detection device for detecting mental fatigue without arbitrariness using a flickering light includes: a light-emitting unit (1) for emitting a flickering visible light； a lighting unit (2) for emitting an infrared ray； an imaging unit (3) for taking an infrared image； and a control unit (4)； wherein: the light-emitting unit (3) presents a test subject a flickering visible light with its frequency changing monotonically and stepwise； the imaging unit (3) captures an image of a region containing an eye of the test subject； the control unit (4) finds a pupil diameter of an image of the eye contained in each frame image of the captured image as time-series data, finds a threshold frequency of the test subject at the time of the measurement based on the variation in the pupil diameter and compares the threshold frequency with a threshold frequency when the test subject has no mental fatigue, thereby determining that whether the test subject has mental fatigue.
