1. An image processing apparatus comprising: an image input unit configured to input an image; a photographing-position information input unit configured to input photographing-position information of a camera that photographs the image, the photographing-position information being information attached to the image input by the image input unit; a determination unit configured to determine whether or not the image is photographed with the camera being rotated in a predetermined direction, with reference to the photographing-position information input by the photographing-position information input unit; a face-detection angle-range information determination unit configured to determine an angle range used in a process of detecting a face from the input image on the basis of an angle at which the camera is rotated, the angle range being limited to a predetermined range that includes the angle at which the camera is rotated, in a case where the determination unit determines that the image is photographed with the camera being rotated in the predetermined direction; and a rotation unit configured to rotate the image by each of a plurality of predetermined degrees within the angle range determined by the face-detection angle-range information determination unit to form a plurality of rotated images; a face detection unit configured to detect a face in the plurality of rotated images rotated by each of the plurality of predetermined degrees by the rotation unit; and an integration unit configured to delete one of overlapping face data in a case where there are overlapping face data in a plurality of face data detected by the face detection unit, and to integrate a plurality of different face data that are left after deleting the overlapping face data as data detected from one image.