A central communication unit 110 and a terminal communication unit 120 are provided with a priority order number storing unit 115 and fifo type storing units, and perform communication according to the priority order of data communication stored in the priority order number storing unit 115. At this time the priority order and the layout of respective devices of control objects are optimized and set through a software processing in a host computer according to the communication cycle and response performance required for the devices of control objects, thereby, the plurality of devices of control objects requiring different communication cycles and response performances can be connected on one network.