A method is provided for extracting an edge included in a color image formed of first pixels expressed by color component values of three colors. The method includes: calculating a lightness value of the first pixel based on the color component values; determining which one of the low, medium, and high lightness regions, the calculate lightness value belongs to; generating an extraction image formed of second pixels corresponding to the first pixels; and extracting an edge based on the pixel value of the extraction image thus generated. For a first pixel whose lightness value belongs to the medium lightness region and the low or high lightness region, a pixel value is calculated from the color component values of three colors, and from the color component values excluding at least one color, respectively. Then, the edge is extracted based on the calculated pixel value of the extraction image.