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 .