1. An authentication apparatus that includes a storage section for storing image data items each indicative of a feature of a specific area of each of a plurality of objects, and authenticates an object in an image as a specific object, using the image data item, comprising: a specific area detection unit configured to detect a specific area of the specific object from the image; a feature information extraction unit configured to extract a feature information item indicative of a feature of the specific area from the specific area of the specific object; an authentication unit configured to compare the image data item and the feature information item, to authenticate the specific object according to a result of the comparison; and a registration determination unit configured, before the feature information item of the specific object is additionally registered in the storage section as the image data item, if one of the plurality of objects is selected as a selected object to which the feature information item of the specific object is to be added, to determine whether or not to additionally register the feature information item of the specific object as an image data item of the selected object, according to a degree of similarity between the image data item of the selected object and the feature information item of the specific object, wherein said registration determination unit determines that the feature information item of the specific object is additionally registered as the image data item of the selected object when the degree of similarity between the image data item of the selected object and the feature information item of the specific object is larger than a registration threshold value set in advance, and wherein the registration threshold value changes according to a registration count of the image data items of the selected object.