In an alertness level estimation device (10), a face image acquisition unit (11) acquires a captured face image representing the face of an occupant in a vehicle. An upper eyelid opening degree calculation unit (12) calculates the degree of opening of an upper eyelid on the basis of the positions, detected from the face image, of an eye inner corner, an eye outer corner, and the highest point of the upper eyelid. When the degree of opening of the upper eyelid is less than a predetermined threshold value, a downward view determination unit (13) determines whether or not the occupant is in a downward view state, on the basis of the position of the highest point of the upper eyelid detected from the face image and the position of an intersection point of a profile line of a lower eyelid and a vertical line drawn downward from the highest point of the upper eyelid. When it is determined that the degree of opening of the upper eyelid becomes less than the threshold value and the occupant is not in a downward view state, an alertness level estimation unit (14) of the alertness level estimation device (10) estimates that the alertness level of the occupant has decreased.
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