PURPOSE: To quantize decision of deterioration of semiconductor laser by reproducing the light emitting waveform of the semiconductor laser based on an electric signal, splitting the waveform with respect to time axis to determine individual area, and comparing the individual area with a reference value, thereby detecting subtle variation of the light emitting waveform.

CONSTITUTION: A laser control drive circuit 3 drives a laser diode(LD) 4 to emit light under the control of a personal computor 1 and the light thus emitted is received by a photodetector 5 where it is converted into an electric signal which is then transmitted through a high rate preamplifier 6 and an analog multiplexer 7 to a digital storage oscilloscope 8. The electric data is taken into a memory in the personal computer 1 as well as a memory in the oscilloscope 8 and that data is employed in the decision of deterioration of the LD 4. The personal computer 1 reproduces the light emitting waveform of the LD 4 and splits the waveform into three sections with respect to time axis and then calculates respective areas which are subsequently compared with a reference value thus determining the extent of deterioration automatically and quantitatively. A reference waveform is produced by multiplying the initial light emitting waveform by a correction coefficient.