A combined air-conditioning and hot-water supply system comprising: a heat source unit provided with a compressor for air conditioning, a first channel switching means, and an outdoor heat exchanger; an indoor unit provided with an indoor heat exchanger and throttle means for air conditioning; a circuit for a hot-water supply heat source provided with a refrigerant - refrigerant heat exchanger and throttle means for the hot-water supply heat source; a branch unit connected to the heat source unit via a high-pressure-side connection pipeline and a low-pressure-side connection pipeline; the branch unit distributing the circulating refrigerant to said indoor unit and said circuit for the hot-water supply heat source; a refrigerating cycle for air conditioning in which said indoor unit and said circuit for the hot-water supply heat source are connected in parallel and connected to said heat source unit via said branch unit; and a refrigerating cycle for hot-water supply in which a compressor for hot-water supply, a heat medium - refrigerant heat exchanger, throttle means for hot-water supply, and said refrigerant - refrigerant heat exchanger are connected in series, wherein said refrigerating cycle for air conditioning and said refrigerating cycle for hot-water supply are connected so that the refrigerant for air conditioning and the refrigerant for hot-water supply exchange heat in said refrigerant - refrigerant heat exchanger, and the branch unit includes a second channel switching means connected to a connection pipeline branching from a connection pipeline that connects the high-pressure-side connection pipeline with one side of the circuit for the hot-water supply heat source, a second connection pipeline connected to one side of the indoor heat exchanger, and a fourth connection pipeline branching from a third connection pipeline that connects the low-pressure-side connection pipeline with a pipeline (134, 136) that connects an other side of the circuit for the hot-water supply heat source with an other side of the indoor heat exchanger, the third connection pipeline thus merging the refrigerants having passed through said indoor unit and said circuit for hot-water supply heat source; wherein a valve means (109, 114) having a variable opening degree is disposed in the third connection pipeline, and said second channel switching means is constituted by a four-way valve in which one outlet port is closed. a heat source unit provided with a compressor for air conditioning, a first channel switching means, and an outdoor heat exchanger; an indoor unit provided with an indoor heat exchanger and throttle means for air conditioning; a circuit for a hot-water supply heat source provided with a refrigerant - refrigerant heat exchanger and throttle means for the hot-water supply heat source; a branch unit connected to the heat source unit via a high-pressure-side connection pipeline and a low-pressure-side connection pipeline; the branch unit distributing the circulating refrigerant to said indoor unit and said circuit for the hot-water supply heat source; a refrigerating cycle for air conditioning in which said indoor unit and said circuit for the hot-water supply heat source are connected in parallel and connected to said heat source unit via said branch unit; and a refrigerating cycle for hot-water supply in which a compressor for hot-water supply, a heat medium - refrigerant heat exchanger, throttle means for hot-water supply, and said refrigerant - refrigerant heat exchanger are connected in series, wherein said refrigerating cycle for air conditioning and said refrigerating cycle for hot-water supply are connected so that the refrigerant for air conditioning and the refrigerant for hot-water supply exchange heat in said refrigerant - refrigerant heat exchanger, and the branch unit includes a second channel switching means connected to a connection pipeline branching from a connection pipeline that connects the high-pressure-side connection pipeline with one side of the circuit for the hot-water supply heat source, a second connection pipeline connected to one side of the indoor heat exchanger, and a fourth connection pipeline branching from a third connection pipeline that connects the low-pressure-side connection pipeline with a pipeline (134, 136) that connects an other side of the circuit for the hot-water supply heat source with an other side of the indoor heat exchanger, the third connection pipeline thus merging the refrigerants having passed through said indoor unit and said circuit for hot-water supply heat source; a connection pipeline branching from a connection pipeline that connects the high-pressure-side connection pipeline with one side of the circuit for the hot-water supply heat source, a second connection pipeline connected to one side of the indoor heat exchanger, and a fourth connection pipeline branching from a third connection pipeline that connects the low-pressure-side connection pipeline with a pipeline (134, 136) that connects an other side of the circuit for the hot-water supply heat source with an other side of the indoor heat exchanger, the third connection pipeline thus merging the refrigerants having passed through said indoor unit and said circuit for hot-water supply heat source; wherein a valve means (109, 114) having a variable opening degree is disposed in the third connection pipeline, and said second channel switching means is constituted by a four-way valve in which one outlet port is closed.