A shading correction is employed for a scanner to correct shading distortion. However, an image corrected with the shading corrective curve has shading noise lines due to the effects of various factors in the producing process of the shading corrective curve. The characteristic of the shading noise is that the each value of any primary color channel, of each pixel in a line is higher or lower than of the adjacent two pixels in other lines, wherein a color channel is one of red, green, or blue channel. Hence, the quality of the image is improved by removing the shading noise detected from the characteristic described above.