In a decentralized control system, a program of the system is described in one form to flexibly cope with changes of the system, and the program developing efficiency, processing performance, and reliability of the system are increased. In configuration, a plurality of control processors and a plurality of devices are connected to a network, the control processors connected to the network are automatically detected and there is determined program execution assignment to possibly equalize the processing load imposed on the control processors. Each control processor executes the program in accordance with the program execution assignment.