A network system includes network relay devices including a master device for administrating the network system, and a member device to be administrated by the master device. When the master device receives an authentication request from an external terminal connected to the network system, the master device performs an authentication processing for authorizing or denying the authentication request. When the authentication request is authorized, one network relay device connected to the external terminal in the network system performs a communication-authorizing processing for authorizing a communication between the external terminal and the one network relay device, and performs a transmission processing for transmitting a communication authorization data to an other network relay device which is not connected to the external terminal in the network system. When the other network relay device receives the communication authorization data, the other network relay device performs the communication-authorizing processing.