A wavelength division multiplexed optical amplifier, enabling elimination of the problems of oscillation and fluctuation in output due to ASE at the time of a rapid change in the number of input wavelengths, provided with a first-stage optical amplifying unit, second-stage optical amplifying unit, a common AGC circuit for common AGC of the first and second optical amplifying units, and a pumping light distribution means for applying pumping light to the first and second-stage optical amplifying units with a predetermined distribution ratio.