The alpha-, sigma- and pi-spectra of quartz in the IR regions were measured under FTIR spectroscopy, and overtone and combination modes perpendicular to the c-axis of quartz were confirmed to be electric vector active by the coincidence of the alpha-spectrum with the sigma-spectrum among the three kinds of spectra. Absorption coefficients of overtone and combination modes intrinsic to the alpha-, sigma-, and pi-spectra of quartz were measured. The thickness of quartz thin sections can be determined by applying the absorption coefficients obtained from the alpha- and sigma-spectra to the polarized IR absorption spectra measured under E perpendicular to the c-axis.