An authentication system stops performing an authentication process on a communication terminal which is in an aborted state. The communication terminal can connect to a network when the authentication thereof is successful. The communication terminal changes between an operational state and an aborted state, and indicates a transition to the aborted state when the communication terminal changes from the operational state to the aborted state. An authentication device repeatedly performs the authentication process, counts a predetermined period for the communication terminal whose authentication has been successful, and performs the authentication process again on the communication terminal when the predetermined period elapses. When a transition to the aborted state is indicated by the communication terminal, the authentication device stops counting the predetermined period for the communication terminal and keeps the authentication of the communication terminal effective.