PURPOSE: To keep constant the pulse amplitude of a laser light by a method wherein a semiconductor laser is driven by the output of a pair of differential transistors driven by a constant current circuit.

CONSTITUTION: The collectors of transistors 11 and 12 are connected to the first input terminal 24 for setting a basic level via collector load resistors 15 and 16, and the emitters of the transistors 11 and 12 are connected to the collector of the transistor 13 whose base is connected to the second input terminal 25 for setting the amplitude of a laser output. An output is applied from the collector of the transistor 11 to the base of the transistor 14 which drives a semiconductor laser 10. The transistors 11 and 12 are conducted alternately according to the presence of a modulation signal applied to the third input terminal 26, and then the amplitude of the collector output of the transistor 11 is determined by the voltage of the input terminal 25 and the values of the resistors 15 and 17; therefore it is not influenced by the level variation of the modulation signal.