An object of the present invention is to provide a spectroscopic method and an apparatus which can measure a trace element accurately with high sensitivity. In order to achieve this object, for example, in Fourier transformation infrared spectroscopy (FT-IR), a reference spectrum and a measurement spectrum including an impurity spectrum are measured in order to obtain a differential spectrum comprising the impurity spectrum and a flat baseline, correction including a frequency shift of the reference spectrum before calculating a differential spectrum, is performed on the reference spectrum. This makes it possible to remove baseline deformation due to phonon absorbance of silicon included in the conventional differential spectrum, and to obtain an infrared absorption spectrum of the substitutional carbon with high accuracy and high sensitivity.