To make APR (Auto Power Reduction) to a required optical fiber transmission line surely function by a simple configuration.

The optical transmission system for bidirectionally transmits main signal light through at least two optical fiber transmission lines between opposing optical transmitters. In the system, when the transmitting direction from one optical transmitter 11A of the opposing optical transmitters to the other optical transmitter 11B is defined as a forward direction and the opposite transmitting direction is defined as a reverse direction, a first optical fiber transmission line 12A transmits the main signal light in the forward direction and transmits OSC (Optical Supervisory Channel) light for supervisory in the reverse direction, and a second optical fiber transmission line 12B transmits the main signal light in the reverse direction and transmits the OSC light for supervisory in the forward direction. The optical transmitter includes an APR control means for stopping the transmission of the main signal light to the optical fiber transmission line which transmits the OSC light when not receiving the OSC light and restarting the transmission of the main signal light when receiving the OSC light.