An air conditioner comprising an indoor unit and an outdoor unit , and an extension pipe connected to the outdoor unit , the indoor unit or outdoor unit comprising a heat exchanger , the heat exchanger comprising: a heat transfer pipe formed of aluminum or aluminum alloy; and a connection pipeline through which a refrigerant flowing in from said heat transfer pipe or a refrigerant flowing into said heat transfer pipe passes, wherein: said connection pipeline has a gas pipeline through which a gas refrigerant flows and a liquid pipeline through which a liquid refrigerant or a gas-liquid two-phase refrigerant flows; each of said gas pipeline and said liquid pipeline is constituted by connecting an aluminum pipeline formed of aluminum or aluminum alloy and a copper pipeline formed of copper and copper alloy using a connection portion ; said connection portion is arranged in a straight part of a descending portion descending from said heat transfer pipe ; and said aluminum pipeline in the straight part of said descending portion is longer than said copper pipeline ; characterized in that the aluminum pipeline is connected to said heat transfer pipe to form the descending portion descending from said heat transfer pipe ; the connection pipeline is connected to the extension pipe by a flare-nut connection of the extension pipe and a flare-nut connection portion of the connection pipeline . a heat transfer pipe formed of aluminum or aluminum alloy; and a connection pipeline through which a refrigerant flowing in from said heat transfer pipe or a refrigerant flowing into said heat transfer pipe passes, wherein: said connection pipeline has a gas pipeline through which a gas refrigerant flows and a liquid pipeline through which a liquid refrigerant or a gas-liquid two-phase refrigerant flows; each of said gas pipeline and said liquid pipeline is constituted by connecting an aluminum pipeline formed of aluminum or aluminum alloy and a copper pipeline formed of copper and copper alloy using a connection portion ; said connection portion is arranged in a straight part of a descending portion descending from said heat transfer pipe ; and said aluminum pipeline in the straight part of said descending portion is longer than said copper pipeline ; characterized in that the aluminum pipeline is connected to said heat transfer pipe to form the descending portion descending from said heat transfer pipe ; the connection pipeline is connected to the extension pipe by a flare-nut connection of the extension pipe and a flare-nut connection portion of the connection pipeline . said connection pipeline has a gas pipeline through which a gas refrigerant flows and a liquid pipeline through which a liquid refrigerant or a gas-liquid two-phase refrigerant flows; each of said gas pipeline and said liquid pipeline is constituted by connecting an aluminum pipeline formed of aluminum or aluminum alloy and a copper pipeline formed of copper and copper alloy using a connection portion ; said connection portion is arranged in a straight part of a descending portion descending from said heat transfer pipe ; and said aluminum pipeline in the straight part of said descending portion is longer than said copper pipeline ; characterized in that the aluminum pipeline is connected to said heat transfer pipe to form the descending portion descending from said heat transfer pipe ; the connection pipeline is connected to the extension pipe by a flare-nut connection of the extension pipe and a flare-nut connection portion of the connection pipeline . <EOS>