A photodiode that can separately detect the intensities of the three wavelength ranges of ultraviolet light of 400 nm or below includes an insulating layer; and a plurality of silicon semiconductor layers having different thicknesses formed on the insulating layer, wherein each of the plurality of silicon semiconductor layers has a low-concentration diffusion layer formed by diffusing one of a P-type impurity or an N-type impurity therein with a low concentration; a P-type high-concentration diffusion layer formed by diffusing a P-type impurity therein with a high concentration; and an N-type high-concentration diffusion layer formed by diffusing an N-type impurity therein with a high concentration, and wherein the P-type high-concentration diffusion layer and the N-type high-concentration diffusion layer formed in a respective one of the plurality of silicon semiconductor layers are arranged to face each other with the low-concentration diffusion layer interposed there between.