An optical amplifier comprising: - a signal beam propagation path including - a beam input unit adapted to receive an input signal beam into which multiple signal beams having respective wavelengths different from each other are multiplexed, - a first beam amplification unit adapted to amplify the input signal beam and to output the amplified input signal beam as an amplified signal beam, - a beam output unit adapted to output the amplified signal beam, and - a variable tilt equalizer which is provided between the first beam amplification unit and the beam output unit and adapted to give loss to the amplified signal beam in accordance with a linear-loss slope, the variable tilt equalizer varying a loss slope value being a value of the linear-loss slope; - a first optical splitter provided between the beam input unit and the first beam amplification unit; - a first photodetector adapted to measure input signal beam power at the beam input unit via the first optical splitter; - an excitation light source (7a, 7b) adapted to excite the first beam amplification unit; - a second optical splitter provided between the variable tilt equalizer and the beam output unit; - a second photodetector adapted to measure output signal beam power output from the variable tilt equalizer via the second optical splitter; - a first control unit adapted to control the excitation light source (7a, 7b); and - a second control unit adapted to control the loss slope value of the variable tilt equalizer, wherein the first control unit is adapted to obtain an operating gain on the basis of the input signal beam power and the output signal beam power, and is adapted to control the excitation light source (7a, 7b) so that the operating gain will become close to a target gain set in advance; and wherein on the basis of the operating gain and a characteristic of gain slope representing wavelength-dependence of gain in the first beam amplification unit, the second control unit is adapted to calculate a gain slope value being a value of the gain slope, and to control the variable tilt equalizer by using the gain slope value as the loss slope value. - a signal beam propagation path including - a beam input unit adapted to receive an input signal beam into which multiple signal beams having respective wavelengths different from each other are multiplexed, - a first beam amplification unit adapted to amplify the input signal beam and to output the amplified input signal beam as an amplified signal beam, - a beam output unit adapted to output the amplified signal beam, and - a variable tilt equalizer which is provided between the first beam amplification unit and the beam output unit and adapted to give loss to the amplified signal beam in accordance with a linear-loss slope, the variable tilt equalizer varying a loss slope value being a value of the linear-loss slope; - a beam input unit adapted to receive an input signal beam into which multiple signal beams having respective wavelengths different from each other are multiplexed, - a first beam amplification unit adapted to amplify the input signal beam and to output the amplified input signal beam as an amplified signal beam, - a beam output unit adapted to output the amplified signal beam, and - a variable tilt equalizer which is provided between the first beam amplification unit and the beam output unit and adapted to give loss to the amplified signal beam in accordance with a linear-loss slope, the variable tilt equalizer varying a loss slope value being a value of the linear-loss slope; - a first optical splitter provided between the beam input unit and the first beam amplification unit; - a first photodetector adapted to measure input signal beam power at the beam input unit via the first optical splitter; - an excitation light source (7a, 7b) adapted to excite the first beam amplification unit; - a second optical splitter provided between the variable tilt equalizer and the beam output unit; - a second photodetector adapted to measure output signal beam power output from the variable tilt equalizer via the second optical splitter; - a first control unit adapted to control the excitation light source (7a, 7b); and - a second control unit adapted to control the loss slope value of the variable tilt equalizer,