PURPOSE: To obtain a pulse signal changing by the timing which is the half of the peak value of the received light quantity of a light receiving transistor without adjusting variable resistance.

CONSTITUTION: This circuit is provided with first and second peak hold circuits PHC 1 and PHC 2 holding the both peak voltages of the output signal of a light receiving element receiving the light from a light emitting element via an encoder, an average value circuit AV determining the average value of the holding voltage of these peak hold circuits PHC 1 and PHC 2 and a comparator comparing the output signals of the light receiving element by defining the output of the average value outputted from this average value circuit AV as a comparison reference and outputting the output signals as encoder output pulse signals.