To decrease the labor for the treating work of a thin oil film by coating the surface of a base body with a thin film containing a semiconductor photocatalyst comprising a semiconductor and exciting the photocatalyst with light so that contaminants in contact with the thin film excited with light are subjected to decomposition, algicidal treatment, microbicidal treatment or deodorization.

The surface of a base body 3 comprising a material having larger buoyant force than gravity or a material treated to have larger buoyant force than gravity is coated with a thin film containing a photosemiconductor photocatalyst 4 comprising a powder or sol containing a semiconductor or metal. The coated photocatalyst 4 is irradiated with light 5 or sun ray 5a to be excited by light 5 of sun ray 5a. Contaminants 1 are brought into contact with the thin film excited with light 5 so that the contaminants 1 are treated by decomposition, algicidal process, bactericidal process or deodorization. Thereby, secondary consideration such as works in the recovery process and diffusion of the environmental pollution becomes unnecessary, and the labor for the treatment of a thin oil film can be decreased.