Two arbitrary points belonging to a single object in an image captured by an camera installed in a vehicle body are extracted as evaluation points, the absolute value representing the difference between the coordinate values of the extracted two points, which are taken in reference to an arbitrary coordinate axis set on the image, is calculated and a time differential value of the absolute value representing the difference between the coordinate values of the two points is determined. Based upon the absolute value representing the difference between the coordinate values and the time differential value, the length of time to elapse before the object containing the two extracted points contacts the vehicle body is calculated.