To provide a Raman amplifier repeater capable of automatically keeping output signal power constant and evenly controlling an output spectrum, and to provide an optical transmission system which uses the same.

In the Raman amplifier repeater wherein amplification of signal beams is performed by Raman scattering, when excited beams are inputted in a transmission line fiber, the power of the signal beams excited by exciting LDs (Laser Diodes) 3a, 3b and 3c is monitored through a plurality of PDs (Photodetectors) 6a to 6d, by using signal beams having monitoring wavelengths whose number is equal to or more than the number of excited wavelengths and equal to or fewer than the number of the signal beams and which are in the signal band of the signal beams to control output power of the excited beams.