A network system includes control devices, network interface units connected to the respective control devices, and operator interface stations each of which stores an address conversion table. Each of the network interface units includes a network-address setting unit to set a virtual identification information when the control device connected thereto is being set for an active system and to set proper identification information when the control device connected thereto is being set for a standby system, and includes an address-conversion-table update instruction unit to request the operator interface stations to update their respective address conversion tables on the basis of the identification information that has been set.