An air conditioner that includes a compressor that compresses a refrigerant, a main outdoor heat exchanger that condenses the refrigerant in a cooling mode and that evaporates the refrigerant in a heating mode, an indoor heat exchanger that evaporates the refrigerant in the cooling mode while condensing the refrigerant in the heating mode, a switch that guides the refrigerant discharged from the compressor to the main outdoor heat exchanger in the cooling mode and that guides the refrigerant discharged from the compressor to the indoor heat exchanger in the heating mode, and a sub outdoor heat exchanger that evaporates a portion of the refrigerant condensed in the main outdoor heat exchanger in a low-load cooling mode and that condenses a portion of the refrigerant discharged from the compressor in a low-load heating mode.