To suppress deterioration of the distance measuring accuracy and the calculation accuracy of relative speed to a preceding vehicle caused thereby by making a decision whether the preceding vehicle is entirely confined within a sweep angle range based on a plurality of distance values being detected at each sweep angle and then calculating the error from the distance to the preceding vehicle thereby estimating the distance to the preceding vehicle.

A distance detection circuit 27 detects the distance to a reflector based on the propagation delay time of a laser light transmitted from a light transmitter 23 and received by a light receiver 25. Based on a distance detected by a distance detection circuit 27 and a sweep angle detected by a laser light sweeping direction detector 33, a signal processing circuit 37 detects a preceding vehicle running on its travel lane. Based on a plurality of distance values detected by a laser light sweeping unit 31 at each sweep angle, a decision is made whether the preceding vehicle is entirely confined within a sweep angle range. When it is confined within the range, distances to the central part and the end part of the preceding vehicle are calculated along with the error thereof and then the distances to the central part is estimated.