A method of limiting I/O access of a client to prevent data in a client connected to the system from being leaked and stolen, the method further canceling the limitation under a predetermined condition even if the client can not communicate with a server is provided. The method comprising the steps of locking I/O access of the client, determining whether the client is connectable to the server via a network, unlocking I/O access of the client in response to a determination of the client being connectable, by authenticating the client by the server, and unlocking I/O access of the client in response to the client not being connectable, by connecting a portable authentication device to the client to authenticate the client by the portable authentication device.