PURPOSE: To reduce the circuit scale, to reduce the number of parts, to improve the characteristic, and to reduce the cost with respect to the circuit which converts the photoelectric current inputted to a photodetector to a voltage and outputs this voltage.

CONSTITUTION: This circuit is provided with a current conversion circuit 10, which uses first and second current mirror circuits consisting of MOS transistors Q1 and 92 and MOS TRs Q3 and Q4 respectively and a MOS TR Q5 for current output from mirror circuits to convert the optical signal made incident on a photodetector D to a current for voltage conversion, and a current/voltage conversion circuit 20 which converts the photoelectrically converted current to a voltage by a diode D2.