1. A learning method for detectors that judge whether a detection target image is a face image, the method comprising the steps of: obtaining face sample image groups each comprising face sample images in which the directions and the orientations of faces are substantially the same, for N types of combinations of the directions and the orientations that are different from each other and predetermined; obtaining N+M types of face sample image groups by generating M other types of face sample image groups wherein combinations of the directions and the orientations are different from the directions and orientations of the N types of face sample image groups, through mirror reversal processing and/or rotation processing in 90 degree increments on at least one of the N types of face sample image groups; and obtaining N+M types of the detectors for which combinations of directions and orientations of faces to be detected respectively correspond to the N+M types of face sample image groups, by causing the respective detectors to learn a characteristic of faces corresponding to the N+M types of face sample image groups.