1. A drowsiness estimating apparatus for estimating a drowsiness level of a subject of drowsiness estimation from among a plurality of drowsiness level candidates, the drowsiness estimating apparatus comprising: an acquisition section that acquires a drowsiness estimation parameter at each of a plurality of acquisition timings; a detecting section that detects an action of the subject of drowsiness estimation; a time-calculating section that calculates a recurrence required time which is the time it takes, from a detection timing at which the action is detected, for a value of the drowsiness estimation parameter acquired after the detection timing to return to a value of the drowsiness estimation parameter acquired before the detection timing; and an estimating section that estimates a drowsiness level of the subject of drowsiness estimation based on the calculated recurrence required time.