1. An image processing apparatus for deciding an image data area of an original from read image data obtained by being read by an image reading apparatus, comprising: an extracting unit configured to extract a plurality of image areas from the read image data; a discriminating unit configured to discriminate whether or not tilt angles of the image areas extracted by the extracting unit are equal; and a deciding unit configured to decide that each of the image areas is an image data area of the original on the assumption that there are a plurality of originals if it is determined as a result of the discrimination by the discriminating unit that one or more tilt angles of the image areas are not equal.