Intended is to lighten the burden on a worker and to improve the manufacturing efficiency. A transparent conductive film or a transparent optical film, as formed on a substrate (W), is irradiated by aline illuminator (3) with a line illuminating light. The line reflected light, as reflected on the transparent conductive film or the transparent optical film, is detected by a camera. The color evaluation value of the reflected light detected is measured, so that the film thickness corresponding to the color evaluation measured is determined by using the film thickness characteristics, in whichthe color evaluation value and the film thickness are correlated.