A signal processing method for performing a signal processing based on a signal output from a photodiode depending on photocharges generated in the photodiode by a light illumination, comprising: performing a signal processing using a photodiode based on a signal output from the photodiode, wherein the photodiode includes a semiconductor layer made of silicon and an insulator layer in contact with the semiconductor layer, wherein the semiconductor layer is provided with a first-conduction-type region and a second-conduction-type region located between the first-conduction-type region and an interface between the semiconductor layer and the insulator layer, the second-conduction-type region having a polarity opposite to the first-conduction-type region, and wherein a thickness of a non-depleted region of the second-conduction-type region is larger than roughness of the interface, and is smaller than a penetration depth of ultraviolet light at which an intensity of the ultraviolet light in a wavelength range of 200-320 nm entering the semiconductor layer is equal to 1/e times the intensity of the same ultraviolet light at an outermost surface of the semiconductor layer, wherein the signal processing is performed by a solid-state imaging system configured to receive light in a wavelength range of 200-1100 nm and to capture an image and comprising: a plurality of pixels integrated in the form of a two-dimensional array, each pixel of the plurality of pixels including the photodiode; and a scan circuit or a charge-signal transfer circuit for sequentially selecting and reading a signal from each of the plurality of pixels, wherein the signal processing is performed by sequentially selecting and reading a signal from each of the plurality of pixels using the scan circuit or the charge-signal transfer circuit. performing a signal processing using a photodiode based on a signal output from the photodiode, wherein the photodiode includes a semiconductor layer made of silicon and an insulator layer in contact with the semiconductor layer, wherein the semiconductor layer is provided with a first-conduction-type region and a second-conduction-type region located between the first-conduction-type region and an interface between the semiconductor layer and the insulator layer, the second-conduction-type region having a polarity opposite to the first-conduction-type region, and wherein a thickness of a non-depleted region of the second-conduction-type region is larger than roughness of the interface, and is smaller than a penetration depth of ultraviolet light at which an intensity of the ultraviolet light in a wavelength range of 200-320 nm entering the semiconductor layer is equal to 1/e times the intensity of the same ultraviolet light at an outermost surface of the semiconductor layer, wherein the signal processing is performed by a solid-state imaging system configured to receive light in a wavelength range of 200-1100 nm and to capture an image and comprising: a plurality of pixels integrated in the form of a two-dimensional array, each pixel of the plurality of pixels including the photodiode; and a scan circuit or a charge-signal transfer circuit for sequentially selecting and reading a signal from each of the plurality of pixels, wherein the signal processing is performed by sequentially selecting and reading a signal from each of the plurality of pixels using the scan circuit or the charge-signal transfer circuit. a plurality of pixels integrated in the form of a two-dimensional array, each pixel of the plurality of pixels including the photodiode; and a scan circuit or a charge-signal transfer circuit for sequentially selecting and reading a signal from each of the plurality of pixels, wherein the signal processing is performed by sequentially selecting and reading a signal from each of the plurality of pixels using the scan circuit or the charge-signal transfer circuit.