A quarantine method and system for allowing a client terminal to connect to a user network. An authentication apparatus recognizes that a communication means of the client terminal has been activated. The authentication apparatus is connected to a quarantine network, to the user network, and to the client terminal. The client terminal is permitted to connect to the quarantine network by confirming a common certificate for the client terminal followed by storing the common certificate in the client terminal. The client terminal is security checked to determine whether each check item of a plurality of check items has a violation. For each check item having a violation, a security measure is performed to improve the check item with respect to the violation. The client terminal is allowed to connect to the user network by confirming a user certificate for the client terminal followed by storing the user certificate in the client terminal.