To provide a photodiode excellent in characteristics.

A mixed crystal layer composed of silicon and germanium wherein ratio of germanium on a junction surface between an N-type semiconductor layer 3 and an intrinsic semiconductor layer 2 is maximum and the ratio of germanium is decreased as leaving from the junction surface is formed in the N-type semiconductor layer 3 on the light incidence side of a photodiode wherein silicon is used as basic semiconductor material. Potential gradient (potential inclination) is made to be generated in the part of the mixed crystal layer of silicon and germanium in the N-type semiconductor layer on the light incidence side. Holes in electron hole pairs generated by incident light 5 in an impurity semiconductor layer on the light incidence side are drifted toward the intrinsic semiconductor layer of silicon by the potential gradient (potential inclination) in the part of the mixed crystal layer of silicon and gelmanium in the light incidence side, and are made it possible to generate a current.