There is provided a device which detects sleepiness of a human being based on the opening and closing motion of an eyelid, in which an occurrence of sleepiness of a subject is detectable with accuracy more sufficient than before even for a subject who keeps from blinking intentionally when sleepiness increases. The inventive sleepiness detecting device comprises an eyelid state detector, detecting an opened/closed state of an eyelid of a subject； a transition time interval detector, detecting sequentially a time interval between transitions between an opened state and a closed state of the eyelid； an eyelid characteristic amount computer, computing an eyelid opening/closing characteristic amount from time series data of the time intervals； a sleepiness judging device, judging that the subject feels sleepiness when the eyelid opening/closing characteristic value falls below a first threshold value or exceeds beyond a second threshold value higher than the first threshold value.
