PURPOSE: To save the energy for air-conditioning of house by a method wherein the heat of the outside air is automatically accumulated in gravles having a large heat capacity and released into room as occasion calls.

CONSTITUTION: In the use for cooling during the summer season, when the difference between the temperature detected by a sensor 22 in gravel layer and the temperature detected by a sensor 23 installed outside the house reaches to the predetermined value during the night, the motor 15 of a ventilating fan rotates to take in the chilly outside air through a window 13 and to discharge through a window 14 into the open air. In the meantime the temperature of the gravel layer 1 lowers to the outside temperature and when the difference between the temperature detected by the sensor 22 and that by the sensor 23 becomes null, the motor 15 stops. And in the daytime, when the cooling is required due to the room temperature rise, by turning a proper switch ON, a ventilating fan 17 rotates to take in the outside air which is cooled by passing through the gravle layer 1 getting cold during the night down to the outside temperature, into the room 5. The air within the room 5 passes through windows 18 and 19 equipped with ventilators to drive out the hot air stagnated in the ceiling through windows 20 and 21 to the outside.