PURPOSE: To confirm the quality assurance of a signal by applying optical loopback automatically on occurrence of a fault and varying a light attenuation factor with an optical attenuator provided to an optical loopback switch.

CONSTITUTION: Each of loopback switches 11-14 is provided to each of optical repeaters 5-8 to locate a fault location on the occurrence of an optical transmission line by using the loopback switches 11-14 so as to bring the optical repeaters 5-8 at both ends of repeaters at the fault location into the optical loopback state. Then an optical attenuator is provided to the optical loopback switches 11-14 of the optical repeaters 5-8 to vary the light attenuation rate by the optical attenuator in the optical loopback state thereby self-diagnosing the quality of signal in the optical repeaters 5-8.