PURPOSE: To detect deterioration in quenching ratio due to secular changes of a laser diode and a driving circuit by superposing a low frequency signal upon the bias current of the laser diode and detecting the low frequency component in a light output.

CONSTITUTION: The PCM signal impressed to an input terminal IN and the output of a low frequency oscillator OSC are passed through an amplifier AMP1 and a capacitor C and superposed upon the DC bias current from a DC bias current source DC and the laser diode LD is driven. Part of its light output is received by a photodiode PD and inputted to a low-pass filter LF through an amplifier AMP2 to control the DC bias current source DC so that the mean value of the light output is constant. On the other hand, only the frequency component of the low-frequency oscillator OSC is extracted from the output of the amplifier AMP2 through a band-pass filter BPF, and when the amplitude of the low frequency component detected by an amplitude detecting circuit DET varies by more than a specific value, a discrimination circuit JUD decides that the laser diode LD deteriorates and generates a warning.