A charge generated in response to the quantity of the incident light into the photodiodes PD is accumulated in an integrating capacitor of an integrating circuit 10, and a voltage V.sub.10 in response to the quantity of the accumulated charge is outputted. The voltage V.sub.10 is inputted into a comparison circuit 20 and is compared with a prescribed threshold voltage therein. When the inputted voltage reaches the threshold voltage, a saturation signal F.sub.1 showing the same is outputted. A certain quantity of charge whose polarity is inverse to that of the charge accumulated in the integrating capacitor of the integrating circuit 10 is injected into the integrating n capacitor by the charge injecting circuit 30 on the basis of the saturation signal. The number of times of the voltage V.sub.10, which is outputted from the integrating circuit 10, reaching the threshold voltage is counted by the counting circuit 40 on the basis of the saturation signal outputted from the comparison circuit 20, wherein a quantity of incident light is detected on the basis of the counted made by the counting circuit 40 and the output voltage value from the integrating circuit 10.
