Abstract: A method for accurately obtaining, in a short period of time, the position and slope of a specific edge line, even when there are many extraneous points caused by dust and other such garbage or electrical noise around the edge line to be obtained. A window covering the area containing the desired edge line is set, and the area inside the window is scanned at a specific interval in a direction crossing the edge line so as to obtain specific image data. The coordinates of points indicating a change in brightness in the scanning direction are obtained as edge points. The area inside the window is divided into a group of rectangular subareas parallel to the edge line to be detected and at a predetermined pitch and size capable of containing the edge line. The subarea containing the greatest number of edge points is obtained, and the edge line is then estimated from the coordinates of the edge points in this subarea containing the greatest number of edge points.