A camera takes an image or images of a region including a vehicle (vehicle-in-front) running in front of a driver's own vehicle. A wheel information acquiring unit acquires the positional information of a wheel or wheels of the vehicle-in-front in the image taken by the camera. More specifically, the boundary between the wheels of the vehicle-in-front and the road surface. The processing for detecting the boundary is done by use of the difference in the brightness between the wheels and the road surface of the vehicle-in-front. A camera attitude correcting unit corrects the detected boundary by use of a self-calibration function, and a road surface condition estimating unit estimates the conditions of the road surface on which the vehicle-in-front is traveling.