When the object detection unit 10 detects the front vehicle X, the speed of the own vehicle 2 is controlled using a detection value of the state of the front vehicle (position, speed) detected by the front vehicle state detection unit 15, and, in the lost state where the front vehicle X is not detected, the speed of the own vehicle 2 is controlled using an estimation value of the state of the front vehicle by the lost front-vehicle state estimation unit 22 based on a condition that a length of the scheduled traveling route of the own vehicle 2 is shorter than a predetermined threshold in the detection target range by the object detection unit 10 as a necessary condition.