1. An image processing apparatus for scanning a plurality of documents that have been placed in a scanning zone capable of being scanned by an image scanning unit and extracting a plurality of items of image data corresponding to respective ones of the plurality of documents, said apparatus comprising: a detection unit adapted to detect a plurality of image areas corresponding to the plurality of documents; a setting unit adapted to set a reference area size from sizes of the plurality of image areas; a comparison unit adapted to compare the sizes of the plurality of image areas and the reference area size; and a decision unit adapted to decide that an image area detected by said detection unit has not been detected correctly if result of the comparison by said comparison unit exceeds a preset threshold value.