A communication system comprising a first communication device and a second communication device that are configured to perform serial communication with each other, either one of the first communication device and the second communication device including a direct-current power source, the first communication device and the second communication device being connected to each other via a communication line that allows electric current supplied from the direct-current power source to flow therethrough from the one communication device to the other communication device and via a reference line that allows the electric current passed through the communication line to flow therethrough from the other communication device to the one communication device, wherein the first communication device comprises: a first transmission circuit comprising a first transmission photocoupler, a conductive or non-conductive state of the first transmission photocoupler being controlled in accordance with data to be transmitted to the second communication device, the first transmission circuit being configured to control a conductive or non-conductive state of a transmission path including the communication line and the reference line in accordance with the conductive or non-conductive state of the first transmission photocoupler; and a first reception circuit comprising a first reception photocoupler, a conductive or non-conductive state of the first reception photocoupler being controlled in accordance with the conductive or non-conductive state of the transmission path, the first reception circuit being configured to detect the conductive or non-conductive state of the first reception photocoupler, thereby receiving data transmitted by the second communication device, the second communication device comprises: a second transmission circuit comprising a second transmission photocoupler, a conductive or non-conductive state of the second transmission photocoupler being controlled in accordance with data to be transmitted to the first communication device, the second transmission circuit being configured to control the conductive or non-conductive state of the transmission path in accordance with the conductive or non-conductive state of the second transmission photocoupler; and a second reception circuit comprising a second reception photocoupler, a conductive or non-conductive state of the second reception photocoupler being controlled in accordance with the conductive or non-conductive state of the transmission path, the second reception circuit being configured to detect the conductive or non-conductive state of the second reception photocoupler, thereby receiving data transmitted by the first communication device, the second reception photocoupler is included in the transmission path, and characterized in that the conductive or non-conductive state of the first transmission photocoupler is opposite to the conductive or non-conductive state of the second reception photocoupler. a first transmission circuit comprising a first transmission photocoupler, a conductive or non-conductive state of the first transmission photocoupler being controlled in accordance with data to be transmitted to the second communication device, the first transmission circuit being configured to control a conductive or non-conductive state of a transmission path including the communication line and the reference line in accordance with the conductive or non-conductive state of the first transmission photocoupler; and a first reception circuit comprising a first reception photocoupler, a conductive or non-conductive state of the first reception photocoupler being controlled in accordance with the conductive or non-conductive state of the transmission path, the first reception circuit being configured to detect the conductive or non-conductive state of the first reception photocoupler, thereby receiving data transmitted by the second communication device, a second transmission circuit comprising a second transmission photocoupler, a conductive or non-conductive state of the second transmission photocoupler being controlled in accordance with data to be transmitted to the first communication device, the second transmission circuit being configured to control the conductive or non-conductive state of the transmission path in accordance with the conductive or non-conductive state of the second transmission photocoupler; and a second reception circuit comprising a second reception photocoupler, a conductive or non-conductive state of the second reception photocoupler being controlled in accordance with the conductive or non-conductive state of the transmission path, the second reception circuit being configured to detect the conductive or non-conductive state of the second reception photocoupler, thereby receiving data transmitted by the first communication device,