A blink detection section generates an eye-closure signal indicating the duration of a period for which the driver closes his/her eyes. An eye-closure count section calculates the respective occurrence frequencies of single eye closures in a prescribed measurement period for a first eye-closure period threshold and a second eye-closure period threshold, based on the eye-closure signal and the respective eye-closure period thresholds. A drowsy state determination section compares the respective occurrence frequencies with the first eye-closure frequency threshold and the second eye-closure frequency threshold to determine which of the drowsiness levels corresponding to the respective thresholds the drowsiness level of the driver is at.