Optical amplifier, and control method therefor

An optical amplifier uses, in a gain medium, a multicore optical fiber having a plurality of cores, and comprises: an input-light power monitor that monitors the optical power of input light to the plurality of cores of the multicore optical fiber; an output-light power monitor that monitors the optical power of medium-passed output light from the plurality of cores that has passed through the multicore optical fiber; a crosstalk monitor that monitors the amount of inter-core crosstalk among the plurality of cores; and a controller that controls the pump-light power of pump light superimposed on the input light to the plurality of cores on the basis of the monitored optical power of input light, the monitored optical power of output light, and the monitored amount of inter-core crosstalk.