PURPOSE: To prevent cracking and film exfoliation and to improve resistance to water, chemicals, corrosion and contamination by specifying the relation between the thickness of a coating film and the temp. of a coated plate and the wavelength of IR having short wavelength for irradiation when the coating material is hardened in a hot stove, and by specifying the temp. of the coated plate at the time of the subsequent concluding hardening.

CONSTITUTION: A plate is coated with a coating material and this coating material is hardened in a hot stove irradiation with IR having short wavelength of 0.7-3μm. This irradiation is carried out until the temp. of the coated plate attains to 100-180°C in case of <25μm thickness of the resulting coating film and to 100-130'C in case of 25-45μm. Thereafter, hardening is carried out at 200-300° temp. of the coated plate. Since a change in film thickness due to complex shape can easily be controlled by a wide range of work conditions, the coating film does not crack or exfoliate and performance peculiar to the coating material can be satisfactorily brought out. A coating film having especially excellent resistance to water, chemicals, corrosion and contamination is obtd.