PURPOSE: To prevent automatically a semiconductor laser from deteriorating beyond a prescribed degree by a construction wherein a driving current circuit for the semiconductor laser is broken in response to an output signal of a comparator which determines the relationship in amplitude between a driving current value of the semiconductor laser and a reference value being settable arbitrarily.

CONSTITUTION: A voltage of Re.IL is generated at a resistor 6 by a driving current IL of a semiconductor laser 4 to be tested which is made luminant by a power set by a variable reference voltage generator 1. When the driving current IL increases with the deterioration of the semiconductor laser 4, an emitter voltage Re.IL of a transistor 5 increases also in proportion to the increase of the current IL. When the level of a deterioration reference level setter 8 is set at a value equivalent to an emitter voltage generated when the deterioration proceeds to a precetermined reference current value, accordingly, the outputs of a level comparator 9 and a flip-flop circuit 10 become H when the value Re.IL increases beyond the set voltage due to the deterioration. Therefore a switch element 3 is opened, and a current ceases to flow to the semiconductor laser 4. By this constitution, various semiconductor lasers being different in the degree of deterioration can be obtained simply.