1. A registration determination device for determining whether to register a candidate image which is a registration candidate of a target image, as a registered image that is registered in advance in order to perform image recognition of a target, the registration determination device comprising: a storage unit configured to store registration information that is at least one of a feature quantity and image data of the registered images in association with discrimination information for discriminating the target; a candidate obtaining part configured to obtain the candidate image; an identifying part configured to indentify discrimination information corresponding to the candidate image obtained by the candidate obtaining part; a feature quantity calculating part configured to calculate a feature quantity of the candidate image obtained by the candidate obtaining part; an in-class variance calculating part configured to calculate, with regard to the candidate image and a registered image in a same class corresponding to the discrimination information identified by the identifying part, an in-class variance of a feature quantity of the candidate image calculated by the feature quantity calculating part and a feature quantity based on the registration information of the registered image, or an in-class variance of a degree of similarity between the feature quantity of the candidate image and the feature quantity of the registered image; an inter-class variance calculating part configured to calculate, with regard to registered images in a plurality of classes respectively corresponding to a plurality of pieces of discrimination information, an inter-class variance of the feature quantity of the candidate image and feature quantities of the registered images based on the registration information of the registered images or the inter-class variance of the degree of similarity between the feature quantity of the candidate image and the feature quantities of the registered images; a variance ratio calculating part configured to calculate a variance ratio which is a ratio between the in-class variance calculated by the in-class variance calculating part and the inter-class variance calculated by the inter-class variance calculating part; and a registration part, wherein when the calculated variance ratio calculated by the variance ratio calculating part satisfies a preset condition, the registration part registers the candidate image as the registered image to the storage unit in such a manner that the registration information of the registered image is associated with the discrimination information identified by the identifying part.