PURPOSE: To prevent the contamination or scratch of a film by applying a film protective agent composed of hot melt powder wax immediately after the baking of topcoat paint to form a wax film.

CONSTITUTION: A car body having an electrodeposition film 2 formed thereto is fed in an intercoat booth through an corrosion-proof process (floor back chipping-resistant material coating process) to be coated with intercoat paint in 15-30μm thickness. A topcoat film 4 is formed to the surface of the intercoat layer 3 in 20-30μm thickness and pigment composed of an inorg. or org. material is contained in a polyester/melamine resin having the same material quality as the intercoat paint. When the topcoat film 4 is formed, a film protective agent is applied to the car body before the car body is fed in an outfitting process. The film protecting agent is powder wax using no solvent or dispersing medium and heated to the m.p. of the powder wax or higher to form a solidified wax film on the topcoat film 4 of the car body through a process of melting →casting → film formation.