PURPOSE: To obtain a laser driving circuit, which can always secure a constant light output and an extinction ratio in an usable temperature range and does not have fluctuation in output waveforms, by flowing a bias current, which is varied with the variation in threshold value of laser light due to temperature change, to a semiconductor laser, controlling an input signal that is applied to one of transistors, thereby compensating for quantum efficiency variation due to the temperature change.

CONSTITUTION: A CML (current mode logic) circuit is used in a high speed laser driving circuit. A thermistor 24 is inserted in a common emitter circuit. The CML is constituted by transistors 10 and 12 and a resistor 14. Said thermistor 24 has replced a part of the emitter resistor of the CML. A capacitor 26 bypasses an AC component. An input signal Sg is applied to the transistor 10, and a reference voltage Vr is applied to the other transistor 12. The resistances of the fixed resistor 14 and the thermistor 24 are suitably selected so as to obtain Sg>Vr. The current value is set so that it is varied with the temperature when a current is switched from the transistor 12 to the transistor 10. The average light output of a laser 20 is detected, and the signal Sg is adjusted so that the output becomes constant.