In an optical transmitter, a laser emits light. A laser driving controller controls driving of the laser by superimposing modulation signals on laser driving signals to generate laser driving superimposed signals and by applying the laser driving superimposed signals to the laser to cause wavelength fluctuations in laser output light to suppress nonlinear optical phenomena during optical fiber transmission. An optical power variable controller variably controls a power of the laser output light. An optical fluctuation compensator suppresses optical fluctuations by monitoring output light from the variable controller to detect optical fluctuations accompanying wavelength fluctuations from monitoring results and by controlling a gain of the variable controller.