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 (31, 41) formed of aluminum or aluminum alloy and a copper pipeline (32, 42) formed of copper and copper alloy using a connection portion (37, 47); said connection portion (37, 47) is arranged in a straight part of a descending portion (33, 43) descending from said heat transfer pipe; and said aluminum pipeline (31, 41) in the straight part of said descending portion (33, 43) is longer than said copper pipeline (32, 42); characterized in that the aluminum pipeline (31, 41) is connected to said heat transfer pipe to form the descending portion (33, 43) 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 (31, 41) formed of aluminum or aluminum alloy and a copper pipeline (32, 42) formed of copper and copper alloy using a connection portion (37, 47); said connection portion (37, 47) is arranged in a straight part of a descending portion (33, 43) descending from said heat transfer pipe; and said aluminum pipeline (31, 41) in the straight part of said descending portion (33, 43) is longer than said copper pipeline (32, 42); characterized in that the aluminum pipeline (31, 41) is connected to said heat transfer pipe to form the descending portion (33, 43) 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 (31, 41) formed of aluminum or aluminum alloy and a copper pipeline (32, 42) formed of copper and copper alloy using a connection portion (37, 47); said connection portion (37, 47) is arranged in a straight part of a descending portion (33, 43) descending from said heat transfer pipe; and said aluminum pipeline (31, 41) in the straight part of said descending portion (33, 43) is longer than said copper pipeline (32, 42); characterized in that the aluminum pipeline (31, 41) is connected to said heat transfer pipe to form the descending portion (33, 43) 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.