To form the photocatalyst thin film exhibiting excellent fouling preventive activity and superior durability by placing a titanium alkoxide solution on the surface of a base material and heating the titanium alkoxide to subject it to thermal decomposition and polymerization to obtain a titanium oxide thin film formed on the base material.

The formation of this photocatalyst thin film comprises: diluting a titanium alkoxide preferably such as titanium tetraisopropoxide, with ethanol or the like, to obtain a solution having an about 0.5 to 20 wt.% alkoxide concn.; vaporizing this solution to generate a vapor; spraying the surface of a base material with the vapor directly or with a gaseous mixture of the vapor and an appropriate carrier gas, wherein the surface of the base material is heated to about 50 to 600°C beforehand; and subjecting a part or the whole of titanium alkoxide placed on the surface of the base material to decomposition and polymerization with heat of the surface of the base material to form a titanium oxide thin film. Thus, the objective photocatalyst thin film having high smoothness, high hardness, hydrophilicity-enhancing properties effected by photoexcitation and organic matter decomposition activity, can be formed on the surface of the base material.