PURPOSE: To improve the stability of an AGC loop and the tracking property by utilizing the output of an error amplifier section and controlling a control signal given to a gain variable amplifier section and a photoelectric conversion section.

CONSTITUTION: A control section 600 outputs a signal proportional to an input Vc from an error amplifier section 500 when the input from the error amplifier section 500 is smaller than a threshold level Vth and a constant signal when larger to a gain variable amplifier section 300 as a 1st control signal and outputs a signal proportional to an input Vc from the error amplifier section 500 when the input from the error amplifier section 500 is larger than the threshold level Vth and a constant signal when smaller to a photoelectric conversion section 100 as a 2nd control signal. Thus, the dead band of a voltage distribution section in an AGC loop due to a diode is eliminated and the stability of an AGC loop and the tracking property are enhanced.