Occurrence of a powder dust explosion is prevented in a photocatalyst coating method of forming a titania layer by injection of titanium powder. Powder of titanium or titanium alloy is injected on a surface of a product to be treated composed of metallic product, ceramic, or mixture thereof, with compressed gas having inert gas as a principal component containing oxygen from 0 to 15% by mass, and the titanium in the powder of titanium or titanium alloy is diffused and oxidized on the surface of the product to be treated to form a titania layer.