1. A subject discriminating apparatus comprising: an image input means for receiving input of a target image, which is a target of discrimination; a first characteristic amount calculating means for calculating first characteristic amounts including directions of gradient vectors or color data, which do not require normalization and are employed to discriminate a predetermined subject, from the target image which is being currently received; a first discriminating means for discriminating whether a candidate of the predetermined subject is included in the target image based on the first characteristic amounts, which are calculated from the target image only, by referring to first reference data, in which learned first characteristic amounts and first discrimination conditions corresponding to the learned first characteristic amounts are defined in advance from sample images of various subjects; a second characteristic amount calculating means for calculating second characteristic amounts, which are normalized and employed to discriminate the predetermined subject, from the candidate of the predetermined subject only when the first discriminating means judges that the candidate is included in the target image; a second discriminating means for discriminating whether the candidate is the predetermined subject based on the normalized second characteristic amounts, by referring to second reference data, in which learned second characteristic amounts and second discrimination conditions corresponding to the learned second characteristic amounts are defined in advance from the sample images; and a control means, for comparing a number of times N 1 that the first discriminating means judges that a candidate is included in the target image to a number of times N 2 that the second discriminating means judges that the candidate is a face, judging whether a ratio of the number of times N 2 to the number of times N 1 is equal to or greater than a predetermined percentage when a number of target images for which discriminating has been performed is equal to a predetermined number, and if the judgment is affirmative, controlling the first characteristic amount calculating means, the first discriminating means, the second characteristic amount calculating means, and the second discriminating means so that the candidate discriminated by the first discriminating means is judged to be the predetermined subject by calculating only the first characteristic amounts from the target images which are subsequently discriminated.