This invention is made to solve the problem that the conventional method cannot correctly determine whether an original is a halftone. To solve this, a method according to this invention detects, from an original, first and second feature pixels respectively exhibiting first and second features. A first feature pixel to first feature pixel distance and second feature pixel to second feature pixel distance are calculated from neighboring first feature pixels and neighboring second feature pixels, respectively. The average of the first feature pixel to first feature pixel distance and the neighboring second feature pixel to second feature pixel distance is calculated as a feature pixel to feature pixel distance. A histogram is generated by the appearance count of only a feature pixel to feature pixel distance for which the same value successively appears. Whether the read original is a halftone is discriminated based on this histogram.