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 is subjected to reflection interference analysis using a Fourier transform infrared spectroscopy. The thickness of the first epitaxial layer is used as a fitting parameter so as to prevent shift between an interference waveform of a resultant 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 a numerically calculated reflection interference pattern in the same wave number range. The thickness of the first epitaxial layer determined during the fitting of the numerically calculated reflection interference pattern is defined as a measured value of the thickness of the first epitaxial layer.