A network system, in which a plurality of computers are connected to a network at a plurality of connectors, and a plurality of address conversion devices are provided in correspondence to the connectors. In each address conversion device, a fixed address on the network corresponding to each address conversion device is stored, while a computer address of one computer connected at one connector corresponding to each address conversion device is acquired and stored. Then, a source address contained in a message transmitted from that one computer to the network given in terms of the computer address is converted into the stored fixed address, while a destination address contained in a message transmitted from the network to that one computer given in terms of the fixed address is converted into the stored computer address.