1. A learning method for causing a face classification apparatus to learn a characteristic feature of faces, wherein the face classification apparatus is an apparatus for classifying whether an input image is a facial image including a face which has a predetermined direction and a predetermined angle of inclination, wherein the characteristic feature of faces is learned by using a machine-learning method using a plurality of facial images for learning, which are different from each other, and each of which includes a face which has the predetermined direction and the predetermined angle of inclination, and wherein the plurality of facial images for learning includes only images of a predetermined facial region, determined based on at least one of the predetermined direction and the type of processing in face detection processing in which the face classification apparatus is used.