To perform cooling and heating by use of geothermal temperature and water supply pressure of city water.

This air conditioning system has: an outside air compression process A for rapidly compressing outside air sucked into an operation cylinder 1 by use of the water supply pressure of the city water, by use of weight of the city water to form high temperature compression air; and an air expansion process B for making the high temperature compression air inside the operation cylinder 1 formed in the outside air compression process heat-exchange with water flowing through a geothermal circulation water passage including an indoor radiator 120 and an underground geothermal radiator 110, and rapidly expanding it. Three processes of an air blowing process C for blowing air through the air expansion process B into a room of a house, an air heating process D for heating indoor air by the water heat-exchanged with the high temperature compression air in the air expansion process B and led in the the indoor radiator 120, and a heated air blowing process E for making the high temperature compressed air formed in the outdoor air compression process A heat-exchange with the water flowing through the geothermal circulation water passage, and gradually blowing it into the room are selectively used in summer and winter.