An air conditioner including a hot gas line for receiving a portion of refrigerant compressed in a compressor, an indoor heat exchanger, an outdoor expansion device for expanding the refrigerant having exchanged heat in the indoor heat exchanger, an outdoor heat exchanger functioning as a condenser in a cooling mode while functioning as an evaporator in a heating mode, and a 4-way valve for receiving a remaining portion of the compressed refrigerant, to guide the refrigerant emerging from the compressor to the outdoor heat exchanger in the cooling mode and to the indoor heat exchanger in the heating mode. The outdoor heat exchanger includes a main heat exchanger section functioning as a condenser in the cooling mode while functioning as an evaporator in the heating mode, and an auxiliary heat exchanger for receiving the refrigerant from the hot gas line in a frosting prevention mode.