PURPOSE: To suppress a rise in the temperature of an attic space and also the rise in an indoor temperature by providing a moisture absorbing-releasing material internally for the attic space ventilated forcedly by a fan and by providing a humidifier supplying moisture to the moisture absorbing-releasing material.

CONSTITUTION: An inlet port 2 is formed for an attic space 1 of a building such as a residence or the like, while an exhaust tube 3 is connected to the attic space. A fan 4 driven by a commercial power source, a solar battery or the like is provided for the exhaust tube 3 and air in the attic space 1 is exhausted from the exhaust tube 3, while outdoor air is led into the attic space 1 from the inlet port 2 so that the attic space 1 can be ventilated forcedly. Besides, a moisture absorbing-releasing material 5 used also as a heat insulating material is lined on the rear of a roof 6, while a heat insulating material 7 is lined on the rear of a ceiling 8, and a humidifier 9 for supplying moisture to the moisture absorbing-releasing material 5 is provided. The moisture absorbing-releasing material 5 is a wood fiberboard, a porous material such as zeolite, or a heat insulating material containing macromolecules having high water absorption properties, for instance.