An air conditioner comprising: a compressor (11, 13) for compressing refrigerant; a hot gas line for receiving a portion of the refrigerant compressed in the compressor; an indoor heat exchanger for allowing the refrigerant compressed in the compressor (11, 13) to exchange heat with indoor air while passing through the indoor heat exchanger; an outdoor expansion device for expanding the refrigerant having exchanged heat in the indoor heat exchanger; an outdoor heat exchanger (70, 80, 90) functioning as a condenser in a cooling mode while functioning as an evaporator in a heating mode, the outdoor heat exchanger (70, 80, 90) allowing refrigerant to exchange heat with outdoor air while passing through the outdoor heat exchanger; and a 4-way valve for receiving a remaining portion of the refrigerant compressed in the compressor (11, 13), to guide the refrigerant emerging from the compressor (11, 13) to the outdoor heat exchanger (70,80, 90) in the cooling mode and to the indoor heat exchanger in the heating mode, wherein the outdoor heat exchanger comprises a main heat exchanger section (70, 80) configured to function as a condenser in the cooling mode while configured to function as an evaporator in the heating mode, and an auxiliary heat exchanger section connected to the hot gas line and configured to receive the refrigerant emerging from the hot gas line in a frosting prevention mode, wherein the main heat exchanger section (70, 80) is configured to exchange heat with outdoor air having exchanged heat with the auxiliary heat exchanger section while passing around the auxiliary heat exchanger section, wherein the auxiliary heat exchanger section is configured to function as a condenser in the cooling mode, to function as an evaporator in the heating mode, and to function as a condenser in the frosting prevention mode, wherein the air conditioner is configured such that, in the frosting prevention mode, the refrigerant emerging from the auxiliary heat exchanger section flows to the main heat exchanger section (70, 80), and evaporates in the main heat exchanger section (70, 80). a compressor (11, 13) for compressing refrigerant; a hot gas line for receiving a portion of the refrigerant compressed in the compressor; an indoor heat exchanger for allowing the refrigerant compressed in the compressor (11, 13) to exchange heat with indoor air while passing through the indoor heat exchanger; an outdoor expansion device for expanding the refrigerant having exchanged heat in the indoor heat exchanger; an outdoor heat exchanger (70, 80, 90) functioning as a condenser in a cooling mode while functioning as an evaporator in a heating mode, the outdoor heat exchanger (70, 80, 90) allowing refrigerant to exchange heat with outdoor air while passing through the outdoor heat exchanger; and a 4-way valve for receiving a remaining portion of the refrigerant compressed in the compressor (11, 13), to guide the refrigerant emerging from the compressor (11, 13) to the outdoor heat exchanger (70,80, 90) in the cooling mode and to the indoor heat exchanger in the heating mode, wherein the outdoor heat exchanger comprises a main heat exchanger section (70, 80) configured to function as a condenser in the cooling mode while configured to function as an evaporator in the heating mode, and an auxiliary heat exchanger section connected to the hot gas line and configured to receive the refrigerant emerging from the hot gas line in a frosting prevention mode, wherein the main heat exchanger section (70, 80) is configured to exchange heat with outdoor air having exchanged heat with the auxiliary heat exchanger section while passing around the auxiliary heat exchanger section, wherein the auxiliary heat exchanger section is configured to function as a condenser in the cooling mode, to function as an evaporator in the heating mode, and to function as a condenser in the frosting prevention mode, wherein the air conditioner is configured such that, in the frosting prevention mode, the refrigerant emerging from the auxiliary heat exchanger section flows to the main heat exchanger section (70, 80), and evaporates in the main heat exchanger section (70, 80).