A representative point corresponding to geometry of the road on which the vehicle is traveling is calculated based on positional information on a stationary target detected by a radar, and based on positional information of the calculated representative points, left and right side corresponding straight lines that correspond to left and right side edges of the road are calculated. With one of the corresponding straight lines that has a most distal end more proximal to the vehicle than that of the other being set as a reference corresponding straight line, information on curving geometry of the road is detected based on positional information on a representative point within a process targeted area that is sandwiched between the left and the right side corresponding straight lines and extends in the traveling direction of the vehicle and is also distal to the most distal end of the reference corresponding straight line.