To provide an underground heat exchanger with high heat exchanging performance, and to provide a heat pump using the same.

An air conditioning system (1) includes a refrigerant circuit (10) in which a first outdoor heat exchanger (25) is connected to an indoor heat exchanger (60) for performing refrigeration cycle, and a heat medium circuit (20) in which an underground heat exchanger (21) is connected to the first outdoor heat exchanger (25). The underground heat exchanger (21) includes a hermetic container (22) for evaporating first heat medium with underground heat. The air conditioning system (1) is configured to perform heating operation, in which the indoor heat exchanger (60) serves as a condenser and the first outdoor heat exchanger (25) serves as an evaporator, meanwhile, the first heat medium is evaporated in the underground heat exchanger (21) and then condensed in the first outdoor heat exchanger (25), for natural circulation between both heat exchangers (21 and 25). The underground heat exchanger (21) is inserted into the hermetic container (22), and includes a warm heat supply pipe (100) in which a second heat medium flows, that exchanges heat with the first heat medium in liquid phase so as to evaporate the first heat medium.