A network system of the present invention includes a computer and a device connected via a network, and a system management device. The computer and the device include, respectively, bridges that encapsulate transmission/reception data transmitted and received to and from each other and transmit and receive the data to and from each other via the network. Each of the bridges includes a control data transmitting means for generating control data for controlling the state of the system based on control auxiliary data issued from the computer or the device and used for controlling the state of the system, and transmitting the control data to the system management device via the network. The system management device includes a system controlling means for controlling the state of the system in accordance with the control data received thereby.