NOVELTY - A semiconductor element consists of an electrode layer (101), semiconductor layer (103), and semiconductor layer (102) having opposing surfaces (I,II). The electrode layer is arranged contacting surface (I) and semiconductor layer (103) is arranged contacting surface (II). The lower limit of electrical potential of the conducting band of semiconductor layer (103) is higher than the hydrogen generation potential, and the upper limit of electrical potential of the valence band of semiconductor layer (102) is lower than the oxygen generation potential.    USE - Semiconductor element is used for hydrogen production systems and methane or methanol production systems (all claimed).    ADVANTAGE - The semiconductor element, in the fabrication of fuel by carbon dioxide reduction utilizing sunlight and fabrication of hydrogen fuel by water decomposition utilizing sunlight, makes it possible to utilize light in a wider range of wavelengths, and, by a low-cost element, to improve the efficiency of carbon dioxide reduction reaction and efficiency of water decomposition hydrogen generation reaction.    DETAILED DESCRIPTION - A semiconductor element consists of an electrode layer, semiconductor layer (103), and semiconductor layer (102) having opposing surfaces (I,II). The electrode layer is arranged contacting surface (I) and semiconductor layer (103) is arranged contacting surface (II). The lower limit of electrical potential of the conducting band of semiconductor layer (103) is higher than the hydrogen generation potential, and the upper limit of electrical potential of the valence band of semiconductor layer (102) is lower than the oxygen generation potential. The band gap of semiconductor layer (103) is greater than the band gap of semiconductor layer (102). INDEPENDENT CLAIMS are included for the following:    (1) hydrogen production system consisting of a transparent container and the semiconductor element arranged in the transparent container such that sunlight passing the transparent container is irradiated to semiconductor layer (103); and    (2) methane or methanol production system consisting of a transparent container and the semiconductor element arranged in the transparent container such that sunlight passing the transparent container is irradiated to semiconductor layer (103).    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of the semiconductor element.    Electrode (101)    Semiconductor layers (102,103)