PURPOSE: To improve reliability by easily specifying an abnormality cause, saving the connection of inspection equipments or the handling time of a reproducing test.

CONSTITUTION: In the transmission device of its own station which is provided with the transmitter and receiver connected with other station via a transmission line, the device is provided with a transmission data extraction means (11, 15) extracting transmission data from a transmission line (14) and making a receiver (16) to receive the data when the transmission data is transmitted from a transmitter (13) to other station in a state that a receiver does not operate, and its own station fault decision output means (11, 17) comparing the transmission data received by the receiver (16) by the transmission data extraction means (11, 15) with the transmission data to be transmitted from the transmitter (13), deciding that a fault exists in its own station when the both of them are different and outputting of its own station fault signal.