A color image is by an optical scanner for conversion into three primary color data corresponding to the respective densities of the three primary colors. An edge detector detects a pixel of an edge based on the three primary color data. A pixel color judging device judges a color of a target pixel based on the three primary color data of the target pixel and its adjacent pixels. Based on edge detection results of the edge detector and pixel color judgment results of the pixel color judging device, detected is a pixel of a black edge. Judgement conditions for the pixel color judgement are established based upon characteristics that depend upon the order of arrangement of color filters in a scanner.
