Abstract: There is provided an optical transmission system that can identify any transmitting stations generating optical beats found within the transmission band being used for optical signals so that any degradation in the quality of the signals being transmitted and adversely affected by the optical beats may be eliminated. The light sources of the transmitting stations giving rise to the optical beats generated in the detector as a result of the differences of the wavelengths of the optical signals coming from the light sources of the transmitting stations and adversely affecting the transmission band being used for the electric signals originating the modulated optical signals are identified by changing the state of polarization of light of the optical signals coming from the light sources of the plurality of transmitting stations.