PURPOSE: To decrease both the number of processing procedure and the processing time, by dividing a tetragonal viewfield into four quarters vertically and horizontally to detect an edge at each divided quarter and deciding the apex of the tetragon from the inclination of edges.

CONSTITUTION: An edge detecting part 10 divides a tetragonal viewfield into four quarters vertically and horizontally to a picture of variabe density in a background 1 of a fixed viewfield including a tetragonal object 3. Then only the outside edge is extracted for each divided quarter part. A contour extracting part 20 extracts 21 the feature points. These feature points are extracted in the form of four feature points, i.e., intersection mark O between edge detecting waveform and divided line as well as for feature points, i.e., points X which are regarded as apexes at the utmost outside. For the point at the utmost outside with which only the coordinates of either single direction is decided, the simultaneous equation is set up with unknown coordinate defined as an unknown quantity to decide the candidate point mark Δ of apex Based on these candiadate point, the inclination and the apex are decided 23 and 24 respectively.