An automobile includes a steering angle detector means (62), a television camera (2) that takes pictures of lanes ahead of the car, and a computer (5) for processing the images taken by the television camera. A reference line is obtained by processing the image of lane markers taken by the television camera. A cubic is approximated from the reference line, and a reference steering angle is determined by the coefficients of the cubic. The reference steering angle is compared with the steering angle detected by the steering angle detector means in order to check for the deviation of the automobile from the lane.