Focusing on pixels that constitute a color image sequentially, color differences Cx and Cy in X and Y directions are found using image data in a mask centering around the focal pixel. For the pixel inwhich the sum of the color differences is not less than a predetermined value, a color difference is found in each of directions E 1 and E 2 that pass through the pixel and correspond to the first quadrant and the second quadrant in the X-Y coordinate system, and it is determined that the color change is generated in the direction having the larger color difference. Then, based on the determination result and the values of the color differences Cx and Cy, an angle (edge code) showing the direction of the color change is calculated.