A semiconductor device is disclosed. The device has a photodiode isolated by element isolating regions (Ia, 14a, 14b) characterized by the following facts: on the principal surface of first semiconductor layer 11 of the first electroconductive type, second semiconductor layer 12 of the second electroconductive type is formed; element isolating region Ia of the first electroconductive type is formed extending through the second semiconductor layer to reach the outer layer portion of the first semiconductor layer so as to isolate the photodiode region; to the outside of the photodiode region isolated by the element isolating regions, moat region E of the second electroconductive type is formed extending through the second semiconductor layer to reach the outer layer portion of the first semiconductor layer; a voltage different from that applied to the second semiconductor layer in the photodiode region is applied to the moat region, and based on the current flowing between the moat region and the second semiconductor layer in the photodiode region, the presence/absence of an inversion layer of the second electroconductive type formed in the outer layer portion of the first semiconductor layer is detected.