PURPOSE: To diagnose most of transmitter faults by generating a diagnostic pulse pattern in its own transmitter starting line service and looping back the pattern from a multiplexing circuit to a demultiplexing circuit so as to check the presence of bit error of demultiplexed information.

CONSTITUTION: At the application of power to a transmitter, each input B of selectors 15, 16 of an active unit 1 is selected, the output of a pulse pattern generator 13 is fed to an MUX 11 and the output of the MUX 11 is fed to a DMUX 12 respectively. Moreover, tri-state buffers 17, 18 are set to a high impedance state respectively, thereby interrupting the active unit 1 and lines 3b, 4a. A signal multiplexed by the MUX 11 is fed to the DMUX 12 via the selector 16 and restored to the original state while being demultiplexed by the DMUX 12 and the result is fed to a bit error detecting circuit 14. The bit error detecting circuit 14 measures the bit error rate and generates an alarm signal when the rate is a threshold level TH or above.