Abstract: A porous material such as paper made of cellulose fiber prepared by a preservation method of the present invention can retain excellent folding endurance after being subjected to accelerated aging treatment performed at a high temperature with high humidity. The method for preserving the porous material includes the steps of placing a porous material made of mainly cellulose fiber in an airtight chamber, evacuating the airtight chamber to reduce the pressure, supplying an ammonia gas into the airtight chamber to thereby neutralize an acidic substance present in the porous material, simultaneously permitting the porous material to absorb the ammonia gas, and supplying an ethylene-oxide-containing gas into the airtight chamber to thereby react with an ammonia present on the surface of and inside the porous material, thus producing an alkaline substance principally consisting of triethanolamine and allowing the alkaline substance to attach to the porous material.