PURPOSE: To obtain an analyzing method using FTIR which enables execution of measurement in a short time, by enabling attainment of a plurality of absorption spectrums by scanning a movable mirror at only one time.

CONSTITUTION: An interferogram is sampled by a frequency being N times higher than the frequency based on a sampling theorem. A first data group is prepared by extracting this sampled interferogram at an interval of data in the number of N-1, starting from/the first data, a second data group is prepared by extracting it at the interval of the data in the number of N-1, starting from the second data, and data groups in the number of N are prepared in the same way. Absorption spectrums in the number of N based on the sampling theorem are determined by subjecting each of these data groups to FFT, and these absorption spectrums are integrated.