An air conditioner comprising: a heat-source side refrigerant circuit in which a compressor, an outdoor heat exchanger, a plurality of intermediate heat exchangers, and a first refrigerant flow-rate controller (25a, 86) disposed between each of the intermediate heat exchangers are connected in series; and a plurality of use-side refrigerant circuits in which each of said plurality of intermediate heat exchangers and a plurality of indoor heat exchangers are connected in parallel, wherein said compressor and said outdoor heat exchanger are disposed in an outdoor unit; said plurality of intermediate heat exchangers are disposed in a relay portion; said plurality of indoor heat exchangers is disposed in each of the plurality of indoor units; in said plurality of intermediate heat exchangers, a heat-source side refrigerant circulating in said heat- source side refrigerant circuit and a use-side refrigerant circulating in said use-side refrigerant circuit are made to perform heat exchange; characterized in that said first refrigerant flow-rate controller (25a, 86) is disposed in the relay portion; a use-side refrigerant channel switching portion capable of selectively switching of said use-side refrigerant circuits disposed in said relay portion is disposed in said relay portion; said use-side refrigerant channel switching portion selectively switches said plurality of use-side refrigerant circuits by connecting any one of said plurality of intermediate heat exchangers to a selected one of said indoor heat exchangers; and said relay portion and each of said plurality of indoor units are connected by two extension pipelines. a heat-source side refrigerant circuit in which a compressor, an outdoor heat exchanger, a plurality of intermediate heat exchangers, and a first refrigerant flow-rate controller (25a, 86) disposed between each of the intermediate heat exchangers are connected in series; and a plurality of use-side refrigerant circuits in which each of said plurality of intermediate heat exchangers and a plurality of indoor heat exchangers are connected in parallel, wherein said compressor and said outdoor heat exchanger are disposed in an outdoor unit; said plurality of intermediate heat exchangers are disposed in a relay portion; said plurality of indoor heat exchangers is disposed in each of the plurality of indoor units; in said plurality of intermediate heat exchangers, a heat-source side refrigerant circulating in said heat- source side refrigerant circuit and a use-side refrigerant circulating in said use-side refrigerant circuit are made to perform heat exchange; said first refrigerant flow-rate controller (25a, 86) is disposed in the relay portion; a use-side refrigerant channel switching portion capable of selectively switching of said use-side refrigerant circuits disposed in said relay portion is disposed in said relay portion; said use-side refrigerant channel switching portion selectively switches said plurality of use-side refrigerant circuits by connecting any one of said plurality of intermediate heat exchangers to a selected one of said indoor heat exchangers; and said relay portion and each of said plurality of indoor units are connected by two extension pipelines.