An optical transmission system includes an optical signal transmitter to output optical signal, a monitoring light transmitter to generate monitoring light, an optical multiplexer to output a multiplexed signal light by multiplexing the optical signal and monitoring light, an optical relay transmission path to amplify the multiplexed signal light by using a Raman amplification effect, an optical demultiplexer to demultiplex the multiplexed signal light, a monitoring light receiver to receive the monitoring light output from the optical demultiplexer, and a pumping light source stopping device to continuously monitor a transmission state of the monitoring light and, when the transmission of the monitoring light is interrupted, to stop emission of the pumping light from the pumping light source to be used for distributed Raman amplification.