A film thickness measurement method conducted by reflection interference analysis using a Fourier transform infrared spectroscopy, comprising the steps of: (a) preparing a measurement target including a semiconductor substrate , and a first epitaxial layer and a second epitaxial layer stacked in this order on the semiconductor substrate and having no difference in refractive index of a real part from the semiconductor substrate , and by using said Fourier transform infrared spectroscopy, measuring an interferogram about said measurement target and measuring an interferogram about said semiconductor substrate to obtain a reference interferogram; (b) calculating a reflection spectrum about said measurement target and a reference reflection spectrum by Fourier-transforming said interferogram about said measurement target and said reference interferogram; (c) calculating a reflection interference pattern by dividing the intensity of said reflection spectrum about said measurement target by the intensity of said reference reflection spectrum; and (d) fitting a numerically calculated reflection interference pattern to said reflection interference pattern , the numerically calculated reflection interference pattern being obtained by numerical calculation based on the structure of said measurement target , wherein in said step (d), the thickness (d b ) of said first epitaxial layer is used as a fitting parameter so as to prevent shift between an interference waveform of said reflection interference pattern containing distortion appearing in a wave number range near an abnormal dispersion range of a refractive index caused by phonon absorption and an interference waveform of said numerically calculated reflection interference pattern in the same wave number range, and the thickness (d b ) of said first epitaxial layer determined during the fitting of said numerically calculated reflection interference pattern is defined as an actually measured value of the thickness (d b ) of said first epitaxial layer , wherein in said step (d), calculation of said numerically calculated reflection interference pattern uses a formula devised by adding a phase shift Δφ corresponding to distortion of said interference waveform containing said distortion to a formula indicating a phase difference generated by a difference between an optical path of infrared light reflecting off a surface of said second epitaxial layer and an optical path of infrared light reflecting off an interface between said first epitaxial layer and said semiconductor substrate. (a) preparing a measurement target including a semiconductor substrate , and a first epitaxial layer and a second epitaxial layer stacked in this order on the semiconductor substrate and having no difference in refractive index of a real part from the semiconductor substrate , and by using said Fourier transform infrared spectroscopy, measuring an interferogram about said measurement target and measuring an interferogram about said semiconductor substrate to obtain a reference interferogram; (b) calculating a reflection spectrum about said measurement target and a reference reflection spectrum by Fourier-transforming said interferogram about said measurement target and said reference interferogram; (c) calculating a reflection interference pattern by dividing the intensity of said reflection spectrum about said measurement target by the intensity of said reference reflection spectrum; and (d) fitting a numerically calculated reflection interference pattern to said reflection interference pattern , the numerically calculated reflection interference pattern being obtained by numerical calculation based on the structure of said measurement target , wherein in said step (d), <EOS>