To provide an air conditioner capable of suppressing waste of artificial energies such as petroleum, gas and electricity, effectively utilizing solar heat and underground heat in the ground to control a room temperature of a house, reducing energy cost and simplifying a structure.

The fresh outside air sucked by a total heat exchange type ventilator 4 mounted in a room of a building is distributed to an underfloor section of a first story of the building, a plurality of U-shaped underground heat recovery pipes 8, 9, 13, 15, 16, 20, 23, 26, 30, 31, 39, 41 are embedded in a foundation 45 under the floor of the first story, blowers 10, 17, 35, 42 are mounted on one end of the underground heat recovery pipes, the air sucked to the underground heat recovering pipes is heated in the underground heat recovery pipes by underground heat in winter, and cooled in the underground heat recovering pipe by the underground cold energy in summer, so that the temperature of the air in the underfloor section of the first story is conditioned, and the temperature-conditioned air is supplied inside of a ceiling of each story through an air supply duct, and further supplied indoors from a louver disposed on the ceiling.