The photo-detecting device includes of a photodetector for detecting incident light, an input J FET for reading the sensing signal from the photodetector, an amplifier for amplifying the signal detected by the input J FET, a feed-back circuit for feeding the output of the amplifier back to the gate of the input J FET through a feed-back capacitor, a reset circuit for resetting the feed-back capacitor by discharging it with a reset MOS FET, and a circuit of a switch and resistor. The same level voltage as the gate voltage of the input J FET is applied to its source through a resistor, and the circuit of the switch and the resistor is connected between the source of the reset MOS FET and the feed-back capacitor. The reset MOS FET and switch are controlled so that the reset MOS FET is turned "on" and "off" while the switch is "on".