To obtain a low-noise optical amplifier circuit which is capable of suppressing the noise light, such as naturally released light, generated by an optical amplifier without using a wavelength filter.

This optical amplifier circuit is constituted by providing the rear stage of the optical amplifier 11 with a Sagnac interferometer 12 connected in series with a cubic optical nonlinear medium 14 and an optical attenuator 15 between the output ports of an optical coupler 13 of 2×2 in such a manner that the output light signal of the optical amplifier is connected to one input port of the optical coupler of the Sagnac interferometer 12 and that the output light signal is taken out of the other input port. The output light signal intensity Pout(t) to the input light signal intensity Pin(t) to the Sangnac interferometer 12 is expressed as Pout(t)=APin(t) [B-cos(CPin(t)] and the value of B is 1 when the branching ratio of the optical coupler is defined as r: 1-r, the insertion loss of the optical attaenuator as α, the nonlinear constant of the optical nonlinear medium as γ and the length as L and A=2αr(1-r), B=(r2+(1-r2))/(2r(1-r)), C=γL(r-α(1-r)) are defined.