Defects in the contour of a target object is detected by obtaining an digital variable-density image of the contour and edges are extracted from the image of the contour so as to sequentially represent the contour. Their directions are measured and the direction of each edge is compared with those of other edges selected according to their distances from the edge under consideration. The differential is indicative of the presence or absence of an indentation or a protrusion in the contour, representing a defect.