Distances between edges which are detected and paired from images which show a driver's face are computed sequentially. Based on a change in the computed distances, a low-probability candidate for a pair of eyelid edges is eliminated. The edge pairs ultimately remaining as a result thereof are detected as upper eyelid edge and lower eyelid edge pairings. It is thus possible to carry out a detection which takes into account not only a feature which is near to an eyelid edge, but also movement as an eyelid. Accordingly, it is possible to accurately detect edges of a driver's eyelids.