An image processing apparatus determines a direction of document placement relative to a scanning direction based on a direction of a shadow region in an input image when a line sensor having a light receiving element performs a scan and an image read from a book document by a reading apparatus is input. When the direction of the binding portion of the document is determined to be along a longitudinal direction of the line sensor, an image of a region read by the light receiving element of the line sensor from reflected light undergoing specular reflection from the document is detected based on the amount of variation in luminance between adjacent pixels in the read image of the book document. Luminance of the image corresponding to the region is corrected.