The present invention relates to an optical amplifier (TDFA) or the like comprising a structure for constantly keeping the power of outputted signal light and gain flatness even when the power or number of channels of input signal light fluctuates.In this optical amplifier, the pumping light outputted from each pumping light source is forwardly or backwardly supplied to an optical amplification fiber. The signal light inputted from an input port is amplified by the optical amplification fiber, and thus amplified light is outputted from an output port. An optical add/drop coupler causes a part of the light outputted from the optical amplification fiber to branch out, whereby its power is monitored by an output light monitor system at two or more wavelengths. According to thus obtained results of monitoring, a controller adjusts the pumping light outputted from each pumping light source.