To track a vehicle regardless of the change of an environmental condition such as unfavorable weather or night with respect to a vehicle detector which detects a low-speed vehicle or a stopping vehicle.

A marker or the like is installed on the road side in the front of a visible camera 11 with respect to detection information outputted by a visible image sensor and a millimeter wave radar device, and the self-detection capability is discriminated by a picture processing part 12 on the basis of a picture of the marker in accordance with the change of the environmental condition to output the result, and a detection information selection processing part 22 automatically selecting the output from a vehicle detector 1 capable of detection is provided, and the vehicle is tracked on the basis of selected detection information, thereby an abnormally running vehicle or a stopping vehicle is detected.