PURPOSE: To treat interference signal measurement and spectrum density distribution calculation as even functions to simplify treatment by calculating a phase error by means of spectroscopy of a spectrum with known width and correcting phase difference between sampling points based on the error.

CONSTITUTION: In a Fourier spectrometer, interference light obtained with light from an object incident to a Michelson interferometer is received by a photoelectric conversion element per sampling point to measure an interference signal F(x) for optical path difference (x), and this interference signal F(x) is treated as an even function so that a spectrum density distribution of light emitted from the object is obtained by Fourier-cosine-transforming the signal in its optical path difference direction. When there is a phase error (s) caused with a constant width for each sampling point, a measured value is substituted into an equation I which is constituted with respect to a spectrum density distribution Bes (ω) obtained when a spectrum of known width [I(ω)=1 Iωs≤ω≤ωe), I(ω)=0 (ω<ωs, ωe<ω)] is incident to obtain the phase error (s). In subsequent spectroscopic measurement, optical path difference (x) for each sampling point is corrected by the phase error (s) to take out the interference signal F(x), and the interference signal F(x) is subjected to Fourier cosine transform to obtain the spectrum density distribution of light.