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 matching data generating unit which generates matching data representing a feature for at least one of the plurality of pieces of biometric information in the at least one biometric image; a mutual biometric information feature amount extraction unit which extracts a mutual biometric information feature amount representing the degree of similarity between two pieces of biometric information of the plurality of pieces of biometric information from the at least one biometric image; a storage unit which, for each of the first predetermined number of registered users, stores the matching data of the registered users and the mutual biometric information feature amount; a selection unit, which, for each of the first predetermined number of the registered users, computes a first selecting score representing the 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, and selects the 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 a matching unit which matches the matching data of the selected registered users with the matching data of the user.