PURPOSE: To improve a fault detecting capability by attaining the detection of a fault in a complicated-shaped or an adjacent and close pattern.

CONSTITUTION: The picture of an object to be inspected in which a synchronous pattern is prepared is fetched in right and left screens by a television camera (step 1). The fetched multilevel pictures 5 and 6 are binarized by a prescribed threshold value so that them, contour part of the pattern can be extracted (step 2). The exclusive logical sum operation of the same coordinate picture elements between those binary pictures 7 and 8 is performed (step 3). A part 9b concerned with a fine difference less than a prescribed reference of an obtained exclusive logical sum picture 9 is eliminated from the original binary pictures 7 and 8 (step 4). Then, obtained binary pictures 10 and 11 are comparison-operated so that the unmatching of the pattern can be detected (step 5).