PURPOSE: To attain the self-diagnosis of a medium connecting device and to prevent a transmission medium from being influenced by a failure detected in a circuit constituting the medium connecting device as the result of diagnosis.

CONSTITUTION: A diagnosis control part 5 for diagnosing an output circuit 1, an input circuit 2 and a control circuit 3 in the medium connecting device and a storage part 4 for storing a diagnostic program are included in the connecting device and a separation part 6 and a change-over switch 7 are also prepared to prevent the execution of the diagnostic program in a case other than a testing state. The control part 5 executes the failure diagnosis of the output circuit 1, the input circuit 2 and the control part 3 based upon an instruction outputted from a start switch 8, and at the time of detecting a failure in any one of the circuits 1 to 3, the transmission medium is disconnected from the output circuit 1 by a disconnection circuit 13 connected between the circuit 1 and the medium.