PURPOSE: To set optionally a saturated exposure amount while suppressing a photodiode area to a small value even if a high resistivity substrate is used for the reduction of current consumption by increasing locally the substrate density at the surrounding part of the photodiode so as to increase the capacity of photodiode.

CONSTITUTION: An impurity density 6 is set higher than the density of the substrate 1 and lower than the density of P-channel diffusion layers 23, 33 of the photodiode. In taking the density of the substrate 1 as NS, the N-channel density of the impurity 6 as ND, and capacitance between the substrate and an N-channel Well and photdiodes as respectively CS and CD, then the ratio of CS/CD is expressed as CD/CS=(ND/NS)1/2 and the capacity is increased by the root of the density ratio to the substrate. Let the density of the ND be ten times that of the NS tentatively, the CD is nearly 3 times the CS and in forming the N-channel Well on the entire face of the photodiode, the saturated exposure amount is triplicated nearly in comparison with the case without the N-channel Well.