A film thickness measuring apparatus of the present invention includes: a light source that emits white light to be irradiated onto a multilayer thin film; a spectroscope that disperses reflected light obtained as a result of irradiating the white light onto the multilayer thin film in order to obtain reflectance spectrums; and a computation section, said computation section including: a setting section that sets a plurality of wavelength ranges for the reflectance spectrums; a first conversion section that obtains wavenumber range reflectance spectrums by re-sequencing, among the reflectance spectrums, reflectance spectrums in the plurality of wavelength ranges set in said setting section at equal intervals, respectively; a second conversion section that converts the wavenumber range reflectance spectrums in the plurality of wavelength ranges obtained in said first conversion section into power spectrums, respectively; and a calculation section that obtains a film thickness of the multilayer thin film based on the power spectrums.