PURPOSE: To detect the life of a semiconductor laser reaching N times as large as the initial current regardless of variation in the initial current value simply by dividing the output of a current detector for detecting the value of current flowing through a semiconductor laser to be inspected at a specified ratio to be compared with a specified set voltage.

CONSTITUTION: A current is fed to a semiconductor laser 2 in such a manner that the output light is kept constant with a control circuit 1 and the voltage corresponding to the current is detected at the node A with a resistance composing a current detector 9. When a switch 3 is closed, the voltage at the connection point B of resistances 10 and 11 forming a divider equals the voltage at the node A. The threshold of a comparator 6 is adjusted through a variable resistance 12 to equal this voltage while the division ratio of the resistances 10 and 11 is selected according to the set multiple N. When the switch 3 is opened, the arrival of the current flowing through a diode 2 at the level N times as large as the initial current is notified of by the lighting of the LED with the high level inversion of the output of the comparator 6 thereby enabling the detection of the life of the semiconductor laser with an easy adjustment of a simple circuit without measuring the initial current value.