To determine a preceding vehicle even when the attitude information with respect to a road of its own vehicle is not accurate.

By a radar device 101 and a radar signal processor 102, for the preceding vehicle A, a distance LAi to its own vehicle and a direction Ai are obtained and preserved in a traveling history preservation means 301. From a GPS device 201, its own vehicle position estimation means 202 and road map data 203, the position of its own vehicle Pi is calculated and preserved in the traveling history preservation means 301. A radius of curvature of road acquisition means 210 calculates the radius of curvature of the traveling road in front and outputs it to a shape comparison means 303. In a preceding vehicle traveling track curvature radius estimation means 302, the set of data at three points of the time is read from the traveling history preservation means 301 and the radius of curvature of the traveling track is calculated and outputted to the shape comparison means 303. In such a manner, the curvature radius of the road and the radius of curvature of the traveling track of the preceding vehicle are compared and whether or not the preceding vehicle travels on the same lane as its own vehicle is judged.