Abstract: This invention provides a method and apparatus to measure the thickness of coating films on running metal sheets to which oil or paint is applied forming coating film such as oil film or paint film. The method to measure coating film thickness employed by this invention is carried out as follows: ultraviolet radiation is projected onto the film formed on the metal sheet; fluorescence emitted due to excitation is detected by a high-sensitivity TV camera; the signal representing the intensity of detected fluorescence is amplified; and the quantity of coating such as oil or paint on the sheet is obtained from the intensity of the amplified signal, which is processed to correct the linear relation between the quantity of coating and the intensity of fluorescence. This invention enables accurate measurement of thickness of coating film such as oil film or paint film coated on metal sheet in an on-line system.