The present invention provides an optical signal noise suppressor and an optical signal noise suppressing method that can decrease noise components to be superimposed on signal light. The optical signal noise suppressor is a device for suppressing noise caused by SBS which is generated when a signal light propagates from a first end side to a second end side of an optical fiber, and comprises a light source section, an optical attenuator, an optical circulator and an optical isolator. A counter light, outputted from the light source section and passing through the optical attenuator, is guided into the optical fiber via the optical circulator. The counter light guided into the optical fiber is interrupted by the optical isolator, and does not reach the signal light source section. The counter light has the same optical frequency components as the SBS light generated by the propagation of the signal light, and propagates in the optical fiber in an opposite direction of the propagation direction of the signal light.