AUTHENTICATION SYSTEM

To provide an authentication technology making it compatible to facilitate inputting of authentication information and to ensure the security. An authentication device connected to a transmitting terminal and a receiving terminal via a communication line, receives a message from the transmitting terminal, receives authentication information from the transmitting terminal, registers the message in association with the authentication information, receives a request with a purport of acquiring the message from the receiving terminal, determines, based on the authentication information received from the receiving terminal and the registered authentication information, whether to authenticate or not, and, if authenticated, transmits the message to the receiving terminal.