Upon receipt of a service use request from a client, an authentication server device reads one or more image information pieces from an image information storage storing multiple image information pieces each containing one or more known symbols, one or more dummy symbols, or both of them, and thereafter creates challenge data using the one or more read image information pieces so that one or more two-dimensional images each containing one or more of the known symbols and one or more two-dimensional images each containing one or more of the dummy symbols can be presented to the user of the client, one image at a time. Upon receipt of response data, the authentication server device judges whether or not the received response data matches the one or more known symbols contained in the challenge data, and approves the service use of the client device if the match is confirmed.