To provide a method of forming a useful organic compound such as methanol or formaldehyde from steam and carbon dioxide for the purpose of effective utilization of carbon dioxide contained in the air or in the combustion exhaust gas as resources, and to provide a system that shows an effect similar to a light reaction performed in the photosynthesis of a plant.

A structural body obtained by distributing an integrated product comprised of a photocatalyst having an excellent oxidizing power and a photocatalyst having an excellent reducing power on an electroconductive substance is disposed in a mixed gas comprising steam and carbon dioxide, and the integrated product is exposed to ultraviolet rays. The photocatalyst having an excellent oxidizing power is an integrated product of titanium dioxide and {a single metal or a single semiconductor element of a divalent metal, a trivalent metal and a tetravalent semiconductor element}, or an integrated product of titanium dioxide and {a mixture of at least two kinds of a divalent metal, a trivalent metal and a tetravalent semiconductor element}. The photocatalyst having an excellent reducing power is titanium dioxide supported by platinum.