PROBLEM TO BE SOLVED: To provide a method for reducing olefin with use of a metal complex catalyst and without parallel use of a chemical reducing agent, more particularly, to provide a titanium oxide/base-metal complex photo-reduction catalyst including combination of titanium oxide that is a solid photocatalyst exhibiting photosensitization action and a base-metal complex compound that becomes a catalytic site.SOLUTION: Using combination of titanium oxide that is a solid photocatalyst exhibiting photosensitization action and a complex compound including a base-metal that becomes a catalytic site, olefin is reduced by being irradiated with UV light.