Abstract: A building air conditioning system using geothermal energy comprising an underground pipe being buried underground for transmitting geothermal energy from the underground to air from the outdoors and for controlling humidity of said air and cleaning said air, a cobble stone layer constituted by filling numerous cobble stones in a space under the floor of a building for introducing air from said underground pipe to heat-exchange this introduced air with geothermal energy stored therein and to control humidity of said air and to clean said air, and air supply means for supply air from said cobble stone layer to an inside of a room of the building.