An air conditioner comprising: an indoor units with an indoor heat exchanger and a first expansion valve; an outdoor units with an outdoor heat exchanger and a second expansion valve; a compressor; a four-way valve disposed at refrigerant pipes through which refrigerant compressed by the compressor flows; and characterized in further comprising: a heating device with an auxiliary heat exchanger; a first pipe coupling the outdoor heat exchanger and the auxiliary heat exchanger, the first pipe having a first valve being opened in cooling mode or in heating mode if the heating device is not used; an indoor-outdoor connecting pipe coupling the outdoor heat exchanger and the indoor heat exchanger; wherein the refrigerant discharged from the indoor heat exchanger can be combined with at least a portion of the refrigerant discharged from the outdoor heat exchanger a second pipe (273, 573) connects the auxiliary heat exchanger to the indoor-outdoor connecting pipe, the second pipe (273, 573) having a second valve being opened in heating mode if the heating device is used; a third pipe (275, 575) coupling the four-way valve and the auxiliary heat exchanger; and a first bypass pipe coupling the first pipe and the compressor, the first bypass pipe having a third valve that opens to allow the outdoor heat exchanger to receive at least the portion of the compressed refrigerant from the compressor; and a parallel pipe connected in parallel to the indoor-outdoor connecting pipe, the parallel pipe having a check valve, wherein the first bypass pipe allows at least a portion of the refrigerant discharged from the compressor to flow to the outdoor heat exchanger without passing through the four-way valve during heating mode. an indoor units with an indoor heat exchanger and a first expansion valve; an outdoor units with an outdoor heat exchanger and a second expansion valve; a compressor; a four-way valve disposed at refrigerant pipes through which refrigerant compressed by the compressor flows; and characterized in further comprising: a heating device with an auxiliary heat exchanger; a first pipe coupling the outdoor heat exchanger and the auxiliary heat exchanger, the first pipe having a first valve being opened in cooling mode or in heating mode if the heating device is not used; an indoor-outdoor connecting pipe coupling the outdoor heat exchanger and the indoor heat exchanger; wherein the refrigerant discharged from the indoor heat exchanger can be combined with at least a portion of the refrigerant discharged from the outdoor heat exchanger a second pipe (273, 573) connects the auxiliary heat exchanger to the indoor-outdoor connecting pipe, the second pipe (273, 573) having a second valve being opened in heating mode if the heating device is used; a third pipe (275, 575) coupling the four-way valve and the auxiliary heat exchanger; and a first bypass pipe coupling the first pipe and the compressor, the first bypass pipe having a third valve that opens to allow the outdoor heat exchanger to receive at least the portion of the compressed refrigerant from the compressor; and a parallel pipe connected in parallel to the indoor-outdoor connecting pipe, the parallel pipe having a check valve, wherein the first bypass pipe allows at least a portion of the refrigerant discharged from the compressor to flow to the outdoor heat exchanger without passing through the four-way valve during heating mode. a heating device with an auxiliary heat exchanger; a first pipe coupling the outdoor heat exchanger and the auxiliary heat exchanger, the first pipe having a first valve being opened in cooling mode or in heating mode if the heating device is not used; an indoor-outdoor connecting pipe coupling the outdoor heat exchanger and the indoor heat exchanger; wherein the refrigerant discharged from the indoor heat exchanger can be combined with at least a portion of the refrigerant discharged from the outdoor heat exchanger a second pipe (273, 573) connects the auxiliary heat exchanger to the indoor-outdoor connecting pipe, the second pipe (273, 573) having a second valve being opened in heating mode if the heating device is used; a third pipe (275, 575) coupling the four-way valve and the auxiliary heat exchanger; and a first bypass pipe coupling the first pipe and the compressor, the first bypass pipe having a third valve that opens to allow the outdoor heat exchanger to receive at least the portion of the compressed refrigerant from the compressor; and a parallel pipe connected in parallel to the indoor-outdoor connecting pipe, the parallel pipe having a check valve, wherein the first bypass pipe allows at least a portion of the refrigerant discharged from the compressor to flow to the outdoor heat exchanger without passing through the four-way valve during heating mode.