PURPOSE: To let automatic adjustment of the optimum level to be performed according to the increase or decrease in input photo signal by a simple circuit constitution of converting the photo signal to electric signal with a photo detector, comparing this with a reference voltage in a comparator and performing level judgement.

CONSTITUTION: When light Pin enters a photo detector 1, output current i1 generates in the photo detector 1. This causes voltages to be generated at both end (a) point and (b) point of a resistance R1. The voltage at the (b) point becomes steady state by the integrating circuit having the time constants of a resistance R2 and capacitor C. This voltage at the (b) point is made the reference voltage of a comparator 2. The voltage at the (a) point is the voltage at the (b) point added with the voltage drop component by the resistance R1. Here the voltage at the (a) point is divided by the resistances R3 and R4 and the voltage at the point P of connection between the resistances R3 and R4 is made the input to the comparator 2. The voltages at the (a) point and (b) point are determined by the current generated by the photo detector 1 and therefore even if the input light changes, it turns out that the discrimination level is set always within the change range of the input signal.