PURPOSE: To prevent the occurrence of dew condensation without lowering of room temperature by forming an air layer on the indoor side of an outer wall for partitioning between the indoor and the outdoor or in the interior of the outer wall, and providing a heating element under the air layer.

CONSTITUTION: An outer wall plate 3 and an inner wall plate 4 are fixed to each other through a connecting material 5. A heat insulating layer 6 is fitted to the back of the inner wall plate 4. An interior plate 10 is erected at a fixed space from the outer wall 1, and an air layer 12 is formed between the indoor side surface 11 of the outer wall 1 and the interior plate 10. Further, a heating element 13 is extended in the cross direction of the outer wall 1 under the air layer 12. Thus, the interior of the air layer 12 is heated above the dew-point temperature of the indoor air by the heating element 13, so that the occurrence of dew condensation can be prevented without lowering of room temperature and the durability of a building can be improved.