To prevent ceiling boards from heating by a method wherein heat insulating members each made up by sticking metal foils or sheets to both sides of an air mat equipped with an air cell are attached to the reverse side of sheathing roof boards.

A heat insulating member 5 comprises of an air mat that is made up of plastic sheet materials with an air cell formed at the inside and by sticking together with a bulge in a spherical shape formed at one side. Then, two air mats are stuck together at their bulge-side while aluminum foils are stuck to both sides at the surface and the reverse thereof. Then, the heat insulating members 5 are installed between purlins 1, 1 with each one end thereof fixed to the underside of the purlin 1, and the reverse side of sheathing roof boards 2 is covered therewith. Thereby, radiant heat emitted from the side of the sheathing roof boards 2 can be prevented from being transmitted to the side of ceiling boards, and temperature rise in dwelling quarters can be restrained.