A communication system capable of shortening a setting time of an ID and reducing an incorrect setting is provided. A writing device sequentially transmits an ID of a plurality of slave devices after transmitting of the ID of a master device. The master device includes a plurality of semiconductor relays provided for each of the plurality of master slave devices for supplying power to the corresponding slave devices. The master device receives the ID of the master device from the writing device, and sets it as its own ID. The master device, every time the ID of the plurality of slave devices is received from the writing device after setting the ID of its own, turns on the semiconductor relays in order. The plurality of slave devices sets the ID received after power supply as its own ID.