To provide an air-conditioning system for a building utilizing geothermy for obtaining an efficient air-conditioning and heating effect in the building by maintaining the desired humidity of air cooled and heated using geothermy and flowing in the building.

This air-conditioning system is provided with a heat storage chamber 7 formed under the floor of the building having an outside air passage around; a heat exchange pipe 16 buried underground so that the opening end communicates with the inside of the heat storage chamber; air passages formed under the floor and in the wall and ceiling of the building and communicating with the heat storage chamber; and a dehumidifier for dehumidifying air supplied into the air passages. Air cooled or warmed by geothermy by the heat exchange pipe and dehumidified by the dehumidifier is allowed to flow in the air passages to regulate room temperature in the building. A filter for imparting antibacterial action or the like to air flowing in the air passages 4a-4c is disposed at or near the dehumidifier.