To attain weight reduction and higher surface pressure strength by drilling a synthetic resin foaming member, filling the drilled hole with cement mortar, and applying cement mortar to the and back surfaces and end surfaces of the synthetic resin foaming member.

A plurality of holes 4 whose diameter is 5 to 30 mm are formed at intervals of 50 to 200 mm by drilling synthetic resin foaming member 3 constituted of polyethylene or the like, and the holes 4 are formed into such a shape that the back side of the synthetic resin foaming member 3 is smaller gradually by enlarging its surface side. The holes 4 are filled with cement mortar 1, and the cement mortar is applied to the front and back surfaces, and end surfaces of the synthetic resin foaming member 3. In the application work of the cement mortar to the synthetic resin foaming member 3, roughness is formed in the surface of the synthetic resin foaming member 3, or metal plain lath 2 or the like is laid for higher adhesive strength, thus it is possible to attain excellent fireproofing and waterproofing and ease of work even by an untrained worker.