First, a plurality of vertices of a contour of an object or of a design are detected from an image. Then, a predetermined number of division points are generated on each of sides connecting the plurality of detected vertices, so as to divide each side of at least one pair of two opposing sides into unequal parts. Then, a plurality of sample points are determined on the basis of straight lines connecting the division points on the two opposing sides to one another, and on the basis of pixel values of the sample points, it is determined whether or not a predetermined object or design is displayed in an area surrounded by the plurality of vertices in the image.