A method of judging a residual film on a sample by an optical measurement, the sample including a first metal film whose reflectance is changed depending on a wavelength of measuring light, and an insulating film formed above the first metal film, and the residual film being a second metal film above the insulating film, the method comprising irradiating the sample with a measuring light so as to measure a change in intensity of light reflected from the sample depending on the wavelength of the measuring light, thereby obtaining a reflectance spectrum curve, and dividing the reflectance spectrum curve into a plurality of wavelength regions so as to judge presence or absence of the second metal film above the insulating film depending on a waveform in each of the wavelength regions of the reflectance spectrum curve.