A laser driver circuit driving a laser diode to generate an optical output corresponding to an input data, and controlling the optical output based on an output current from a photodiode that detects the optical output. The output current is converted into a voltage signal, whose amplitude is detected after filtering the voltage signal by a low-pass filter. A reference signal corresponding to the input data is generated, and its amplitude is detected. A modulator for generating a modulation current regulates the amplitude of the modulation current according to the difference between the two detected amplitudes.