To judge whether or not a remaining metal film exists on an dielectric multilayered film by simple computation, without using optical constants of the metal.

The maximum value of a spectral reflectance for each wavelength on the occasion of changing the film thickness of the uppermost layer of a dielectric multilayered film is found beforehand as a maximum reflectance represented by curve MLmax. Since the spectral reflectance changes from curves A50 to A5, as the thickness of a remaining metal film becomes thinner if the metal film remains on the dielectric multilayered film, and it becomes curve A0 if the metal film disappers and does not remain, it is judged that an object of judgment has a remaining metal film, if a peak wavelength where the spectral reflectance of the multilayered film to be an object of judgment becomes peak is found in a designated wavelength region, and if the reflectance of the object of judgment exceeds a maximum reflectance at the peak wavelength, and that a remaining metal film does not exist, if the reflectance of the object of judgment is equal to or smaller than the maximum reflectance at the peak wavelength.