1. An authentication apparatus comprising a CPU or hardware logic circuits configured to form the following components: an extraction part configured to extract a feature of a face from a face image of a user; a plurality of classifiers configured to identify the user by determining whether or not the feature extracted by the extraction part matches a feature of a face stored in a storage apparatus; a deriving part configured to derive a specific condition based on at least one of the feature extracted by the extraction part and information for authenticating the user; a specific classifier configured to narrow down the number of user candidates to be authenticated, wherein the derivation of the specific condition comprises one of a first process of comparing a total number of users registered in the storage apparatus with a predetermined threshold, and a second process of determining, by the specific classifier, whether or not the feature extracted by the extraction part matches the feature of the face stored in the storage apparatus; a selecting part configured to select at least one of the plurality of classifiers based on the specific condition, wherein when the derivation of the specific condition comprises the first process, the selecting part selects a first classifier among the plurality of classifiers if the total number of users registered in the storage apparatus is less than the predetermined threshold, and the selecting part selects a second classifier among the plurality of classifiers if the total number of users registered in the storage apparatus is greater than or equal to the predetermined threshold, the second classifier being different from the first classifier, wherein when the derivation of the specific condition comprises the second process, the selecting part selects a third classifier among the plurality of classifiers based on the user candidates narrowed down by the specific classifier; and an authentication part configured to authenticate the user when the selected classifier determines that the feature extracted by the extraction part matches the feature of the face stored in the storage apparatus.