A drowsiness determination device includes an input unit that receives vehicle information indicating the amount of variation in an attitude of a vehicle with respect to a road surface and head information indicating the amount of movement of a head of an occupant with respect to a predetermined position and a determination unit that determines whether the occupant is drowsy, on the basis of the vehicle information and the head information. The determination unit determines that the occupant is drowsy, on the conditions that when the vehicle attitude varies, the occupant head moves in the same direction as the direction in which the vehicle attitude has varied and that a delay time between the timing when the vehicle attitude has varied and the timing when the occupant head has moved is longer than a predetermined time.