TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Arrangement: The semiconductor element is soaked in the electrolyte aqueous solution in the transparent container. The methane or methanol production system has a gas blowing inlet which introduces a source gas containing carbon dioxide.    TECHNOLOGY FOCUS - ELECTRONICS - Preferred Arrangement: The lower limit of electrical potential of the conducting band of semiconductor layer (103) is higher than the reaction electric potential of methane or methanol. The band gap of semiconductor layer (103) is greater than the band gap of semiconductor layer (102). The photocatalyst layer contacts the surface facing semiconductor layer (102) of semiconductor layer (103). The co-catalyst is discretely supported by the surface of photocatalyst layer. The side surface of the laminate of semiconductor layers and said electrode layer is sealed with an insulating material.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The electrode contains platinum, gold, silver, copper, nickel, indium, rhodium, palladium, gallium, zinc, titanium, cobalt, iron and/or iridium. The semiconductor layers contain titanium, tantalum, tungsten, zinc, tin, iron, copper, vanadium, zirconium, neptunium, molybdenum, cadmium, antimony, cobalt, oxide, sulfide, selenide, silicon carbide, silicon, gallium phosphide, copper-indium-gallium-selenium, and/or copper-indium-selenium. Semiconductor layer (102) comprises a semiconductor having a band gap of 0.5 eV or more. Semiconductor layer (103) comprises a semiconductor having a band gap of 1.5 eV or more. The oxide used for semiconductor layer (103) is doped with potassium and strontium. A photocatalyst is discretely supported on the surface of semiconductor layer (103). The photocatalyst is anatase type titanium oxide, rutile type titanium dioxide, and/or brookite type titanium oxide. A co-catalyst is directly supported on semiconductor layer (103). The co-catalyst is platinum, silver, copper, gold, nickel oxide, ruthenium oxide, and/or copper oxide.