To provide a method of forming a useful organic compound such as formaldehyde from water vapor and carbon dioxide for the purpose of effectively utilizing carbon dioxide contained in the air or in the combustion exhaust gas as resources, and 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 water vapor 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 copper oxide (II), an integrated product of titanium dioxide, iron oxide (III) and cerium oxide (IV), or an integrated product of titanium dioxide, iron oxide (III) and platinum-supported cerium oxide (IV). The photocatalyst having an excellent reducing power is titanium dioxide supported by platinum.