To provide an underground heat using air conditioning system enabled in more efficient heat exchange even if a distance from a building is increased.

In a range of L2=5-20 m from a quay 1 as a bank protection structure, underground heat exchange wells 2 are excavated at 1-2 m of intervals, and reciprocation underground heat exchange pipes 3 are installed in the underground heat exchange wells 2 to exchange heat with the ground around the underground heat exchange wells 2. A distance to the building 6 as an air conditioning target is set to be approximately L1=50-1,000 m. Liquid such as an antifreeze solution serving as a heating medium is circulated in piping 4, and thermal energy obtained by the heat exchange in the positions of the underground heat exchange wells 2 is used for cooling in the building in summer and heating and hot water supply in the building in winter via a heat pump 5 or directly without the interposition of the heat pump 5. In the vicinity of the quay 1, by being affected by a high tide and a low tide of the sea, the fluctuation of the underground water level is more remarkable compared to other sections, and thus, the retention of heat is small so as to enable highly efficient heat exchange.