PURPOSE: To protect a semiconductor laser from an excess current by branching the exciting current of a semiconductor laser on the basis of the result detecting the radiated light from the semiconductor laser.

CONSTITUTION: An optical beam emitted from the back side of a laser diode LD is incident to a photodiode PD. The photodiode PD is connected to a resistor 14 for sensing connected between the base and the collector of a transistor 12 for sensing. The photodiode PD is applied with reverse bias to flow a photocurrent iPH through a sensing resistor 14 when the light is incident. When the photocurrent iPH exceeds the maximum photocurrent iPHmax of the maximum allowable amount, the base voltage of the transistor 12 is determined in accordance with RD=(VB-VL)/iPHmax for the value of the resistor 14 for sensing, thereby increasing from the voltage drop VL of the laser diode LD. Thus, the branching transistor 11 is conducted, the current iS of part of the exciting current iE is received instead of the laser diode LD. Accordingly, the current iL=iE-iS flowed thrugh the laser diode LD is limited to the maximum allowable value.