To provide an air conditioner suppressing waste consumption of artificial energy such as petroleum, gas and electricity, effectively using solar heat and underground heat to control room temperature of a house, reducing cost and having a simple structure.

Outside air sucked by a total heat exchange type ventilation fan mounted to an indoor side of a building is sent to a space under a first floor of the building, and a plurality of underground heat recovery pipes formed to have a U-shape are embedded in a foundation bottom plate under the first floor, and an air blower is mounted to one end of the underground heat recovery pipe. Air sucked by the underground heat recovery pipes is warmed in the underground heat recovery pipes by underground heat in winter, and is cooled in the underground heat recovery pipes by underground heat in summer, to control the temperature of air in the space under the first floor. The air of which temperature is controlled is supplied to inside of a ceiling on each floor via an air supply duct, and is supplied to the indoor side by a louver provided on the ceiling.