An optical amplifier that improves transmission quality by driving an excitation light source according to a change in the level of an input optical signal and by compensating for a change in the characteristics of the excitation light source. An optical amplification section includes an amplification medium doped with an active substance for optical amplification and an excitation light source for emitting excitation light. A constant-gain control section finds out target excitation light power to be set according to a change in the level of an input optical signal on the basis of a correlation between a gain and excitation light power to obtain a target gain, finds out a target drive signal for making the excitation light source emit the target excitation light power on the basis of a correlation between a drive signal for driving the excitation light source and excitation light power, and exercises constant-gain control over the optical amplification section by a feedforward control system.