Abstract: A board-like porous material is coated with a polymerizable monomer over its upper and lower surfaces and is thereby immediately impregnated with the monomer. The material is then placed into shallow containers. A specified number of the containers containing the material are stacked, a lid is placed over the uppermost container, and the containers are hermetically closed by being held together by fastening means. The bottom wall of the container and the lid are each of a hollow structure and include a plurality of parallel partitions providing hot air passages arranged in parallel and having open front and rear ends. Finally, hot air is supplied to the passages to heat the containers, whereby the polymerizable monomer impregnating the material is vaporized, diffused through the material and then polymerized without application of pressure to the coated porous material.