A new technique for determining the pore size distributions (PSDs) from the melting and freezing curves of water confined in pores, thermal porosimetry termed thermoporosimetry, has been established by combining some physical properties determined by a preceding study with iterative optimization of the layer of nonfreezable water. By applying the technique to the pore structure analysis of 12 kinds of commercial silica gels, the cumulative pore volumes, the surface areas, and the peak radii of the PSD curves were determined from both the DSC melting and freezing curves of freezable pore water and were found to be in quite good agreement with those of the PSD curves obtained via nitrogen gas adsorption-desorption and mercury porosimetry. Thermoporosimetry has the potential for determination of the PSD over a wider range of pore sizes than nitrogen gas adsorption-desorption.