1. An image processing apparatus comprising: an acquisition unit configured to acquire a read image which has been obtained by reading a plurality of originals by a reading apparatus and includes an original region corresponding to each of the plurality of originals; a specifying unit configured to specify a plurality of objects as a group of objects each of which corresponds to a group of pixels having luminance values smaller than a predetermined luminance value in the read image acquired by said acquisition unit; and a tilt determination unit configured to determine a first tilt of a first group of objects and a second tilt of a second group of objects specified by said specifying unit; a distance determination unit configured to determine a distance between the first group of objects and the second group of objects; a decision unit configured to decide a candidate region corresponding to each of the plurality of originals based on the first tilt and the second tilt determined by said tilt determination unit and the distance determined by said distance determination unit.