A communication device for making bidirectional multiplex communication with a plurality of slave communication devices (002a-d), comprising: a first port connected to one end of a transmission line to which the slave communication devices (002a-d) are connected; a second port connected to the other end of the transmission line; and a current detection unit connected to the first port and the second port; a current detection/modulation unit connected to the first port and second port performing data transmission/reception over the transmission line via each of the first port and second port, and wherein a power voltage for operating is supplied from the communication device to the slave communication devices (002a-d) via the transmission line, and the current detection unit detects a change in the quantity of current flowing in at least one of the first port and the second port, and detects an occurrence of disconnection on the transmission line on the basis of the change in the quantity of current, and wherein when transmitting data to the communication device, each of the slave communication devices (002a-d) changes the quantity of current in the transmission line depending on the data to be transmitted at a mutually-different timing, the communication device comprises a table indicating a correspondence between each of the slave communication devices (002a-d) and a physical position on the transmission line to which each of the slave communication devices (002a-d) is connected, when receiving data from each of the slave communication devices (002a-d), the communication device registers a current of the quantity of current detected by the current detection unit in the table in association with the slave communication device from which the data is received, and the communication device specifies a disconnected place on the transmission line on the basis of a change between the current widths corresponding to the slave communication devices (002a-d) registered in the table. a first port connected to one end of a transmission line to which the slave communication devices (002a-d) are connected; a second port connected to the other end of the transmission line; and a current detection unit connected to the first port and the second port; a current detection/modulation unit connected to the first port and second port performing data transmission/reception over the transmission line via each of the first port and second port, and wherein a power voltage for operating is supplied from the communication device to the slave communication devices (002a-d) via the transmission line, and the current detection unit detects a change in the quantity of current flowing in at least one of the first port and the second port, and detects an occurrence of disconnection on the transmission line on the basis of the change in the quantity of current, and wherein when transmitting data to the communication device, each of the slave communication devices (002a-d) changes the quantity of current in the transmission line depending on the data to be transmitted at a mutually-different timing, the communication device comprises a table indicating a correspondence between each of the slave communication devices (002a-d) and a physical position on the transmission line to which each of the slave communication devices (002a-d) is connected, when receiving data from each of the slave communication devices (002a-d), the communication device registers a current of the quantity of current detected by the current detection unit in the table in association with the slave communication device from which the data is received, and the communication device specifies a disconnected place on the transmission line on the basis of a change between the current widths corresponding to the slave communication devices (002a-d) registered in the table.