A method for controlling a flow of a RADIUS (Remote Authentication Dial-In User Service) protocol, where the method allows a plurality of RADIUS servers to share a load in a RADIUS system in which a RADIUS client of the RADIUS system can swiftly receive a response message from a RADIUS server being responsive to a request message from the RADIUS client. The method includes the steps of allowing a RADIUS server to transmit a notification message to a RADIUS client on the basis of a packet format in the RADIUS protocol, the notification message having information relating to an available or unavailable state of the RADIUS server, and allowing the RADIUS client to check the available or unavailable state of the RADIUS server from the notification message received from the RADIUS server.