An air conditioning system, comprising: a compressor compressing a refrigerant; an indoor heat exchanger condensing the refrigerant discharged from the compressor in a heating operation; an outdoor heat exchanger evaporating the refrigerant condensed in the indoor heat exchanger; and a heater to provide heat to the refrigerant; characterized in that the heater includes: a heating chamber formed with a passage in which the refrigerant flows, a heat transferring part having one surface contacting the refrigerant flowing in the passage, two electrodes spaced apart from each other on the other surface of the heat transferring part, the other surface being opposed to the one surface; a plurality of carbon nanotube heating elements disposed on the other surface of the heat transferring part to be spaced away from each other, having both ends connected to the two electrodes, respectively, and wherein the sum of a contacting area of the plurality of carbon nanotube heating elements and the heat transferring part is determined to be at least 50% or more of the contacting area of the heat transferring part and the refrigerant flowing in the passage, and an insulating member insulating the electrodes and the carbon nanotube heating elements. a compressor compressing a refrigerant; an indoor heat exchanger condensing the refrigerant discharged from the compressor in a heating operation; an outdoor heat exchanger evaporating the refrigerant condensed in the indoor heat exchanger; and a heater to provide heat to the refrigerant; characterized in that the heater includes: a heating chamber formed with a passage in which the refrigerant flows, a heat transferring part having one surface contacting the refrigerant flowing in the passage, two electrodes spaced apart from each other on the other surface of the heat transferring part, the other surface being opposed to the one surface; a plurality of carbon nanotube heating elements disposed on the other surface of the heat transferring part to be spaced away from each other, having both ends connected to the two electrodes, respectively, and wherein the sum of a contacting area of the plurality of carbon nanotube heating elements and the heat transferring part is determined to be at least 50% or more of the contacting area of the heat transferring part and the refrigerant flowing in the passage, and an insulating member insulating the electrodes and the carbon nanotube heating elements. a heating chamber formed with a passage in which the refrigerant flows, a heat transferring part having one surface contacting the refrigerant flowing in the passage, two electrodes spaced apart from each other on the other surface of the heat transferring part, the other surface being opposed to the one surface; a plurality of carbon nanotube heating elements disposed on the other surface of the heat transferring part to be spaced away from each other, having both ends connected to the two electrodes, respectively, and wherein the sum of a contacting area of the plurality of carbon nanotube heating elements and the heat transferring part is determined to be at least 50% or more of the contacting area of the heat transferring part and the refrigerant flowing in the passage, and an insulating member insulating the electrodes and the carbon nanotube heating elements.