To provide the edge position of a work from image information in contactless state by detecting from which boundary point to which boundary point it is to be expressed in a single straight line or circular arc from the dot sequence of found boundary points and finding the equation of the circular arc and straight line forming the contour of the work.

An image input device 4 generates two-dimensional arrangement by inputting digitized image data. Next, by finding difference between the image, which is previously acquired by a video camera 3, of a work installer 1 mounting no work and the input image containing the current work, a differential image almost removed excepting for the work part is generated. Further, the contour of the work is detected from the differential image, it is discriminated from the coordinate value sequence of the contour whether the respective parts of the work contour are straight lines or circular arcs, and data are prepared. From these data, edge emphasizing processing is first performed to the area near the dot sequence of respective detected edges, and the exact positions of edges are calculated by edge detecting processing such as Hough transformation.