A controlled apparatus periodically transmits state information indicating the state of the apparatus, and when the state has been changed, transmits state information indicating the state after the change. Upon receiving state information from the controlled apparatus, a control apparatus, when not requesting a change of state in the controlled apparatus, returns the state information to the controlled apparatus, and when requesting a change of the state in the controlled apparatus, changes state parameters in the state information that correspond to the state to be changed to required values and transmits the state information after the change to the controlled apparatus as a control command. The controlled apparatus, upon receiving the control command from the control apparatus, changes to a state in accordance with the state parameters that follow the change and transmits the state information indicating the state after the change to the control apparatus.