PURPOSE: To make constant the photo output, by comparing the changing part of the signal component of the photo detection output of the semiconductor laser with reference value after obtaining with the clamp circuit and controlling the injection current to the semiconductor laser.

CONSTITUTION: The photo output of the semiconductor laser 1 is detected with the photo-voltage conversion section 3, and the detected output consisting of the bias component VB and the signal component Vp is fed to the clamp circuit 5. The circuit 5 thus constituted clamps the potential to E corresponding to the bias component VB, and the voltage of E+VD is fed to the input terminal IN 3 of the circuit 5 by taking the forward voltage of the diode as VD. In this case, by taking the voltage E equal to the DC potential fed to the input terminal IN2, only the detection of the changing part of the signal component VP greater than the bias component VB can be possible. After that, the injection current of the laser 1 is controlled by using the changing part, and the change in the threshold value of the laser 1 due to the mark rate change and the temperature change can be avoided