A communication device that performs bidirectional multiplex communication between a plurality of slave communication devices and the communication device is provided with: a first port connected to one end section of a transmission line to which the slave communication devices are connected; a second port connected to the other end section of the transmission line; and a current detection unit connected to the first port and the second port. An operation voltage is supplied from the communication device to the slave communication devices, a change of the quantity of a current flowing in the first port and/or the second port is detected by means of the current detection unit, and on the basis of the current quantity change, occurrence of disconnection in the transmission line is detected.