An air conditioning system using ground heat, the system comprising: one or more indoor units through which a coolant flows for conditioning indoor air; at least one outdoor unit communicating with the indoor units via a plurality of pipes through which the coolant flows; wherein the air conditioning system further comprises: an outdoor heat exchanger where heat exchange occurs between the coolant and a circulation medium; an overcooler configured to further cool down the coolant heat-exchanged at the outdoor heat exchanger; a ground heat exchanger connected with the outdoor heat exchanger of the outdoor unit, and laid under the ground to allow heat to be exchanged between ground heat and a circulating medium circulating through the ground heat exchanger; and a circulating pipe configured to connect the outdoor heat exchanger with the ground heat exchanger to guide the circulating medium heat exchanged with the coolant in the outdoor heat exchanger to the ground heat exchanger, at least one portion of the circulating pipe laid under the ground; a plurality of auxiliary heat sources (400, 450, 460) installed on one side of the outdoor unit to assist heat exchange of the outdoor heat exchanger, wherein two or more auxiliary heat sources of the plurality of auxiliary heat sources (400, 450, 460) are simultaneously used, or one of them is selectively used, and wherein the plurality of auxiliary heat sources (400, 450, 460) are installed on the circulating pipe to heat or cool down the circulating medium. one or more indoor units through which a coolant flows for conditioning indoor air; at least one outdoor unit communicating with the indoor units via a plurality of pipes through which the coolant flows; wherein the air conditioning system further comprises: an outdoor heat exchanger where heat exchange occurs between the coolant and a circulation medium; an overcooler configured to further cool down the coolant heat-exchanged at the outdoor heat exchanger; a ground heat exchanger connected with the outdoor heat exchanger of the outdoor unit, and laid under the ground to allow heat to be exchanged between ground heat and a circulating medium circulating through the ground heat exchanger; and a circulating pipe configured to connect the outdoor heat exchanger with the ground heat exchanger to guide the circulating medium heat exchanged with the coolant in the outdoor heat exchanger to the ground heat exchanger, at least one portion of the circulating pipe laid under the ground; a plurality of auxiliary heat sources (400, 450, 460) installed on one side of the outdoor unit to assist heat exchange of the outdoor heat exchanger, wherein two or more auxiliary heat sources of the plurality of auxiliary heat sources (400, 450, 460) are simultaneously used, or one of them is selectively used, and wherein the plurality of auxiliary heat sources (400, 450, 460) are installed on the circulating pipe to heat or cool down the circulating medium. an outdoor heat exchanger where heat exchange occurs between the coolant and a circulation medium; an overcooler configured to further cool down the coolant heat-exchanged at the outdoor heat exchanger; a ground heat exchanger connected with the outdoor heat exchanger of the outdoor unit, and laid under the ground to allow heat to be exchanged between ground heat and a circulating medium circulating through the ground heat exchanger; and a circulating pipe configured to connect the outdoor heat exchanger with the ground heat exchanger to guide the circulating medium heat exchanged with the coolant in the outdoor heat exchanger to the ground heat exchanger, at least one portion of the circulating pipe laid under the ground; a plurality of auxiliary heat sources (400, 450, 460) installed on one side of the outdoor unit to assist heat exchange of the outdoor heat exchanger, wherein two or more auxiliary heat sources of the plurality of auxiliary heat sources (400, 450, 460) are simultaneously used, or one of them is selectively used, and wherein the plurality of auxiliary heat sources (400, 450, 460) are installed on the circulating pipe to heat or cool down the circulating medium.