A method of non-invasively determining a concentration of an in-vivo component such as blood sugar level (glucose) of a subject is provided. An absorption spectrum of the subject is measured by use of near-infrared light. The concentration of the in-vivo component is determined by use of the absorption spectrum of the subject and a calibration curve. The calibration curve is prepared by determining a plurality of difference absorption spectra that are differences between a plurality of near-infrared absorption spectra of a living body and a reference absorption spectrum selected from the near-infrared absorption spectra, determining a plurality of synthetic absorption spectra, which are obtained by synthesizing each of the difference absorption spectra with a previously measured reference absorption spectrum of the subject, and performing a multivariate analysis with use of the obtained synthetic absorption spectra.