In order to enhance reliability of a determination regarding whether a preceding vehicle is running in the same lane as a following vehicle when only one of the preceding vehicle and the following vehicle is in a curved region of a road, a distance and an apart-angle of the preceding vehicle, and an amplitude of a reflective wave are detected, and it is determined, on the basis of the detected apart-angle and amplitude of the reflective wave, whether the preceding vehicle is in the same lane as the following vehicle.