A vehicle-mounted detecting device and corresponding method for detecting a distance between the vehicle and an object outside the vehicle. The detecting device extracts edge intersection points from images of individual frames photographed by a camera. A set of edge intersection points for which the relative position in images does not change between frames is then specified as an observation point group; the movement of the specified observation point group in images is observed; and pitching by the vehicle is detected. Any two points with differing heights on an object present in an image are extracted as feature points, and after the effect of pitching is eliminated from the feature points, the position of a vanishing line in the image is calculated based on the ratio of the velocities of the feature points in images after the length between the two feature points and the effects of pitching are eliminated.