the present invention is to provide a laser driving circuit having a comparably short time constant , accordingly , can follow fluctuations with a short time interval . the circuit of the present invention provides a feedback transistor in addition to a driver transistor for driving the laser diode and a photodiode for detecting a portion of the optical output of the laser diode . the feedback transistor controls , by receiving the information derived from the photo current generated in the photodiode , the current flowing in the driver transistor and the laser diode . the time constant of the feedback loop , i . e ., the laser diode , the photodiode , the feedback transistor , the driver transistor and the laser diode , is far smaller than that of the apc loop . therefore , the feedback control for the short time constant phenomena may be carried out .