A new hydrophilic method for achieving a high-performance adsorbent by impregnating silica into pores of activated carbon has been proposed. In the process, silica-gel was synthesized in the pores of activated carbon after inserting a sodium silicate solution into the pores. Three types of activated carbon were tested to produce hydrophilic activated carbon and to investigate the hydrophilicity by measuring the adsorption isotherms of water vapor and pore structure characteristics. The proposed method worked effectively for adding hydrophilicity to activated carbon, since all of adsorption isotherms of the material treated with this method shifted to a lower region of water vapor pressure than that of the raw material. The shift to a lower relative pressure of adsorption isotherm was maximum when the concentration of sodium silicate solution was 1 mass %.