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),