A method for measuring a film thickness distribution of a first thin film of a first wafer with thin films by reflection spectroscopy, the first wafer having the first thin film formed on a surface of a substrate and a second thin film formed on a surface of the first thin film, the method comprising: calculating a profile P1 by a simulation, the profile P1 indicating wavelength dependence of a reflectance of the first wafer with respect to a light at wavelengths not less than a wavelength region of visible light, the first wafer being an object to be measured; calculating a profile P21 by a simulation, the profile P21 indicating wavelength dependence of a reflectance of a second wafer with respect to a light at wavelengths not less than a wavelength region of visible light, the second wafer having thin films including a first thin film having the same thickness as in the first wafer and a second thin film that is t [nm] thinner or thicker than a set film thickness T2 of the second thin film of the first wafer; calculating a profile P31 (= P21 - P1) of a difference between the calculated profiles P1 and P21 and obtaining a wavelength λ1 observed when the calculated profile P31 of the difference becomes zero; and selecting a waveband including the obtained wavelength λ1 as a waveband of a light for use in the film thickness distribution measurement by the reflection spectroscopy that prevents the reflectance from varying due to variation in film thickness of the second thin film, wherein the film thickness distribution of the first thin film is measured by the reflection spectroscopy in a manner that a surface of the first wafer is irradiated with a light to selectively measure only reflected light at the selected waveband of reflected light from the surface of the first wafer, or in a manner that the surface of the first wafer is irradiated with a light at the selected waveband to measure all reflected lights of the reflected light from the surface of the first wafer. calculating a profile P1 by a simulation, the profile P1 indicating wavelength dependence of a reflectance of the first wafer with respect to a light at wavelengths not less than a wavelength region of visible light, the first wafer being an object to be measured; calculating a profile P21 by a simulation, the profile P21 indicating wavelength dependence of a reflectance of a second wafer with respect to a light at wavelengths not less than a wavelength region of visible light, the second wafer having thin films including a first thin film having the same thickness as in the first wafer and a second thin film that is t [nm] thinner or thicker than a set film thickness T2 of the second thin film of the first wafer; calculating a profile P31 (= P21 - P1) of a difference between the calculated profiles P1 and P21 and obtaining a wavelength λ1 observed when the calculated profile P31 of the difference becomes zero; and selecting a waveband including the obtained wavelength λ1 as a waveband of a light for use in the film thickness distribution measurement by the reflection spectroscopy that prevents the reflectance from varying due to variation in film thickness of the second thin film,