When a controlled device has changed the value of a parameter, the controlled device transmits the result of the change as updated information UPDATE to controllers by multicast, and aside from this, also transmits synchronization data SYNC in each synchronization period by multicast. Upon detecting a reception error of those data, the controller requests, from the controlled device, the information of the parameter which has been changed in one or more synchronization periods including and after the synchronization period subsequent to the last synchronization period in which the controller could normally receive all of the packets, and corrects the parameter value in the controller based on the current value of the changed parameter transmitted from the controlled device in response to the request.