PURPOSE: To contrive the influence of deflection due to the tare weight, moisture absorbing, and the like of thermal insulation material, not to be directly exerted on ceiling material by setting the thermal insulation material via an air layer on the upper side of the porous ceiling material.

CONSTITUTION: Ceiling material 1 is supported by mounting a peripheral edge on reversed-T-shaped bars 17 suspended via hangers 16 from suspension bolts 15. After that, on the upper surface peripheral-edge of the ceiling material 1 consisting of porous metallic plates, thermal insulation material 2 is mounted on the upper side via spacers. Between the ceiling material 1 and the thermal insulation material 2, an air layer 4 is formed, and even if the thermal insulation material 2 is deflected by static pressure due to the tare weight, moisture absorbing, and airconditioning, a load due to the influence is directly loaded on the central section of the ceiling material 1 and the material 1 is set not to be deflected.