Erroneous determination of an image caused by color shift is prevented. Image data is divided in a sub-scanning direction into a first region where color shift hardly occurs and a second region where color shift easily occurs. There are detected a first color line drawing extending in a main-scanning direction in the first region, a second color line drawing extending in the main-scanning direction and having a width larger than that of the first color line drawing in the second region, and a third color line drawing extending in the sub-scanning direction in the image data. On the basis of the detected first, second, and third color line drawings, it is determined whether a document is a color document or not.