In order to identify an optical cable for optical communication from a
remote place, a Sagnac interferometer including two strands of an optical
fiber is formed in the optical cable, and a worker in the remote place
applies a disturbance of a popping sound to an optical cable to be
identified. The disturbance applied by the worker is detected and
regenerated in the form of a sound. The optical cable can be easily
identified by comparing the regenerated signal and the disturbed signal
in the remote place to thereby prevent an incorrect optical cable from
being cut. In addition, the optical cable can be more precisely
identified by selecting a different light detecting frequency component
in accordance with environment conditions.