To suppress deterioration in transmission quality caused by polarization dependency.

An optical transmission system comprises an optical transmitter and an optical receiver. The optical transmitter includes a polarization control part and a reception part. The polarization control part changes a polarization state by performing variable control of an operation frequency within a frequency range having polarization followability of the optical receiver to polarization of signal light on the basis of a notified monitoring result. The reception part receives the notified monitoring result of transmission quality of the signal light. The optical receiver has a monitor control part including a monitor part and a notification part. The monitor part monitors the transmission quality of the received signal light. The notification part transmits the monitoring result to the optical transmitter.