An optical transmission system using Raman optical amplification, which is configured in a WDM-PON topology where a signal light between an optical line terminal and each of optical network units is multi/demultiplexed at a WDM. The optical line terminal supplies continuous lights having wavelengths to the optical network units through an optical fiber from the optical line terminal. The continuous lights are used for an uplink signal. In addition, the optical line terminal outputs a pumping light having a wavelength which is used to excite lights of the continuous lights, to the optical fiber from the optical line terminal.