A communication system having a controller and a plurality of controlled devices (130, 132, 134) in communication with said controller, characterized in that: consecutive addresses are allocated respectively to each of said plurality of controlled devices; said controller is configured to transmit a request of device information to each of said plurality of controlled devices after switching on a power supply, and in response to receive addresses of said controlled devices, device information including information indicating whether it is applicable to multicast communication, from said controlled devices, said device information received from each of said controlled devices being stored in a device management table, to check that the addresses of the controlled devices capable of multicast communication are consecutive by referring to the device management table, and if the addresses are not consecutive, to transmit an address modification request to devices for which an address modification is required in order for the addresses to become consecutive, to transmit, after transmitting said address modification request, a data request including a particular value indicating multicast communication to the plurality of controlled devices managed by the particular value among the plurality of controlled devices, wherein each of the controlled devices, other than a controlled device which firstly transmitted a response to said data request is configured to check a content of the response to the data request from other controlled devices when receiving the data request, to check an address of the controlled device which transmits the response included in the response, and, if the value of the address immediately precedes a value of its own address, to transmit a response including its own address to the controller. consecutive addresses are allocated respectively to each of said plurality of controlled devices; said controller is configured to transmit a request of device information to each of said plurality of controlled devices after switching on a power supply, and in response to receive addresses of said controlled devices, device information including information indicating whether it is applicable to multicast communication, from said controlled devices, said device information received from each of said controlled devices being stored in a device management table, to check that the addresses of the controlled devices capable of multicast communication are consecutive by referring to the device management table, and if the addresses are not consecutive, to transmit an address modification request to devices for which an address modification is required in order for the addresses to become consecutive, to transmit, after transmitting said address modification request, a data request including a particular value indicating multicast communication to the plurality of controlled devices managed by the particular value among the plurality of controlled devices, wherein each of the controlled devices, other than a controlled device which firstly transmitted a response to said data request is configured to check a content of the response to the data request from other controlled devices when receiving the data request, to check an address of the controlled device which transmits the response included in the response, and, if the value of the address immediately precedes a value of its own address, to transmit a response including its own address to the controller.