It is detected whether an automobile is running in a direction departing from a traffic lane, has already departed from the lane, or is running in a direction recovering from departure, by mounting on the automobile a steering angle detecting device for detecting an actual steering angle based on steering wheel operation of a driver, a television camera for taking pictures of the lane and a computer for processing the pictures taken by the television camera. The television camera is used to take pictures of markers on the lane while the automobile is running on the lane, and video signals are input into the computer. The computer is then used to process the input signals, thereby setting a virtual reference line forming a cubic curve approximating the set virtual reference line, calculating a virtual standard steering angle from a coefficient of a function representing the cubic curve, and comparing the steering angle detected by the detecting device based on the steering wheel operation of the driver with the virtual standard steering angle calculated by the computer.