For a read image having reflection of a light source, which occurs when a document is read, the reflection of the light source of the read image is corrected by the steps of: selecting an RGB color component having minimum average values of each RGB color component of a read image among RGB color components of an read image having reflection of a light source that occurs when a document is read; calculating the amount of change of luminance values of pixels of the read image derived from the reflection, from the values of pixels of the color components selected in the read image; and correcting the reflection of the light source in the read image by correcting the luminance values of pixels of the read image using the calculated amount of the change of the luminance values of the pixels.