To form a titania film on the surface of a base without detriment to decorative properties and thereby ensure a superb corrosion resisting performance by applying an aqueous amorphous titanium peroxide solution to the surface of a base formed of magnesium or its alloy and heating the solution at a specified temperature or below to form a transparent titania film.

A 0.1-2 μm thick titania film 2 is formed on the surface of a substrate 1 of a base. In this case, when the titania film 2 is thinner than 0.1 μm, a sufficient corrosion-resisting effect cannot be obtained and when the film 2 is thicker than 2 μm, the film 2 is easily peeled. In addition, magenium or an alloy containing magnesium and aluminum, zinc or the like is used as a material for the substrate 1. Further, an aqueous solution or an aqueous viscous material solution containing an amorphous titanium peroxide is applied to the surface of a magnesium alloy substrate 1 using a spray process or the like. Then the coat is thermally dried at below 200°C after being dried at normal temperatures to obtain the magnesium substrate 1 containing an amorphous titania film. Thus it is possible to form the transparent titania film 2 having outstanding adhesive properties with substrate 1 and qualitative uniformity/ surface flatness.