An amount of charges consonant with the intensity of the light entering photodiodes is generated, and the level of the charges is determined by a charge level determination circuit. Based on this determined charge level, a capacitance setting circuit sets a capacitance of an integrating capacitor unit in an integrating circuit. Thereafter, in the integrating circuit, the charges generated by the photodiodes are integrated in the integrating capacitor unit, and a voltage having a value consonant with the amount of the integrated charges is output. When background light is strong and the overall intensity of incident light is high, a comparatively large capacitance is set for the variable capacitor unit of the integrating circuit, and the intensity of the incident light is detected without saturation. When background light is weak and the overall intensity of incident light is low, a comparatively small capacitance is set for the variable capacitor unit of the integrating circuit, and the intensity of the incident light is detected at high sensitivity, regardless of the surrounding conditions.
The present invention aims at providing a photodetector which can improve each of the dynamic range, S/N ratio, and speed of light detection. A photodiode PD.sub.m,n generates electric charges by an amount corresponding to the incident light intensity, and accumulates thus generated electric charges into a junction capacitance part. An electric charge amount level determining circuit 10.sub.m,n determines the level of amount of electric charges generated by the photodiode PD.sub.m,n and accumulated in the junction capacitance part. In an integrating circuit 20.sub.m, the capacitance value of an integral capacitance part 21 is set according to the result of electric charge amount level determination, the electric charges inputted from the photodiode PD.sub.m,n by way of a switch SW1.sub.m,n are accumulated into the integral capacitance part 21, and a voltage V.sub.20 corresponding to the amount of thus accumulated electric charges is outputted. The voltage V.sub.inp fed to a non-inverting input terminal of an amplifier A in the integrating circuit 20.sub.m is set greater in a second period during which the integral capacitance part 21 of the integrating circuit 20.sub.m accumulates electric charges than in a first period during which the junction capacitance part of the photodiode PD.sub.m,n accumulates electric charges.
