An optical amplifier is provided which can suppress, without measuring signal beam power at individual wavelengths, wavelength-dependence of gain with respect to a signal beam into which multiple signal beams having respective wavelengths different from each other are multiplexed. The optical amplifier (100) according to the present invention can suppress wavelength-dependence of gain by giving loss in accordance with a linear-loss slope to an amplified signal beam. The optical amplifier is provided with a variable tilt equalizer (11) for varying a loss slope value representing the slope of the loss slope and a tilt control unit (22) for controlling a loss slope value of the variable tilt equalizer.