The triple point temperature of water condensed in silica gel, that is chosen as a porous solid, is abserved as 24°C below the normal triple point temperature, by thermal analysis method. The mean radius of capillaries in silica gel, which is observed after the triple point depression, is 19 A°. Using this value, the triple point pressure is obtained as 2.27 mm Hg less than the normal triple point pressure.