PURPOSE: To improve pitching resistance and the quality of external appearance, by holding a workpiece to be coated just after being coated with cross-linking curing resin paint in a NOX atmosphere of predetermined concentration at a predetermined temp. for a predetermined time.

CONSTITUTION: A workpiece to be coated is coated using cross-linking curing resin paint. Said workpiece to be coated just after being coated with said paint is held in an atmosphere having NOX concentration ≥200ppm at a temp. of 70W150°C for a time of 15W40min. Hence, the density of cross linking on the surface of the coating film is enhanced by curing treatment to increase the hardness of said surface. Thus, pitching resistance and the quality of external appearance are improved.