PURPOSE: To transmit internal information, to diagnose the line connection device of an opposite station and to easily grasp fault conditions by mutually transmitting and receiving special codes.

CONSTITUTION: At the time of a diagnostic mode, a control part 20 performs changeover from a code transmission part 22 to the transmission part 26 by a selector 211 and transmits a code for starting at the time of transmitting the internal information. In the opposite station 4, when the code for starting is detected in a code detection part 37, the control part 310 is informed. The control part 310 performs the changeover from the code transmission part 32 to 36 by the selector 311, encodes the various kinds of fault information from an internal information part 38 and transmits them through an U/B conversion part 33 to the opposite station 2. In the device 2, the internal information is gathered in an internal information processing part 29 and a result is transmitted to the control part 20. Thus, the faults of a called station side device, a terminal equipment and a cable, etc., can be diagnosed mutually between the opposite stations.