1. A biometric authentication device comprising: a biometric information acquiring unit which generates at least one biometric image representing a plurality of pieces of biometric information of a user; a storage unit which, for each of a first predetermined number of registered users, stores matching data representing a feature for at least one of the plurality of pieces of biometric information and a mutual biometric information feature amount representing a degree of similarity between two pieces of biometric information of a plurality of pieces of biometric information; a processor configured to generate the matching data from the at least one biometric image; extract the mutual biometric information feature amount from the at least one biometric image; for each of the first predetermined number of the registered users, compute a first selecting score representing a level of similarity between the plurality of pieces of biometric information of the user and the plurality of pieces of biometric information of the registered users, based on the mutual biometric information feature amount of the user and the mutual biometric information feature amounts of the registered users, select a second predetermined number of registered users less than the first predetermined number in order of decreasing the level of similarity based on the first selecting score for each of the first predetermined number of registered users; and match the matching data of the selected registered users with the matching data of the user.