A combined air-conditioning and hot water supply system comprising: a refrigerant circuit formed by connecting, by pipes, at least one heat source unit including a compressor and a heat source-side heat exchanger, at least one indoor unit including an indoor-side heat exchanger and an indoor-side expansion device, at least one hot water supply unit including a refrigerant-water heat exchanger and a hot water supply-side expansion device, and a branch unit including a gas-liquid separator that supplies a gaseous refrigerant to the indoor unit and/or the hot water supply unit heating a heat exchange target with which the gaseous refrigerant exchanges heat, and supplies a liquid refrigerant to the indoor unit and/or the hot water supply unit cooling a heat exchange target with which the liquid refrigerant exchanges heat, and a refrigerant flow control device that controls passage of a refrigerant to the indoor unit and the hot water supply unit; and a control processing device, characterized in that the control processing device is configured to determine that there is a possibility of freezing of water in a water-side flow path in the refrigerant-water heat exchanger, while the hot water supply unit is stopped, to start a freezing prevention process by performing a control that decreases a pressure difference between a pressure in a refrigerant-side flow path in the refrigerant-water heat exchanger and a pressure in a pipe which is connected to the refrigerant-side flow path in the refrigerant-water heat exchanger via the hot water supply-side expansion device, if the control processing device determines that there is a possibility of freezing of water in a water-side flow path in the refrigerant-water heat exchanger, by opening the hot water supply-side expansion device to a predetermined opening degree or by causing the refrigerant flow control device to communicate the refrigerant-side flow path in the refrigerant-water heat exchanger with a gas supply side of the gas-liquid separator. a refrigerant circuit formed by connecting, by pipes, at least one heat source unit including a compressor and a heat source-side heat exchanger, at least one indoor unit including an indoor-side heat exchanger and an indoor-side expansion device, at least one hot water supply unit including a refrigerant-water heat exchanger and a hot water supply-side expansion device, and a branch unit including a gas-liquid separator that supplies a gaseous refrigerant to the indoor unit and/or the hot water supply unit heating a heat exchange target with which the gaseous refrigerant exchanges heat, and supplies a liquid refrigerant to the indoor unit and/or the hot water supply unit cooling a heat exchange target with which the liquid refrigerant exchanges heat, and a refrigerant flow control device that controls passage of a refrigerant to the indoor unit and the hot water supply unit; and at least one heat source unit including a compressor and a heat source-side heat exchanger, at least one indoor unit including an indoor-side heat exchanger and an indoor-side expansion device, at least one hot water supply unit including a refrigerant-water heat exchanger and a hot water supply-side expansion device, and a branch unit including a gas-liquid separator that supplies a gaseous refrigerant to the indoor unit and/or the hot water supply unit heating a heat exchange target with which the gaseous refrigerant exchanges heat, and supplies a liquid refrigerant to the indoor unit and/or the hot water supply unit cooling a heat exchange target with which the liquid refrigerant exchanges heat, and a refrigerant flow control device that controls passage of a refrigerant to the indoor unit and the hot water supply unit; and a gas-liquid separator that supplies a gaseous refrigerant to the indoor unit and/or the hot water supply unit heating a heat exchange target with which the gaseous refrigerant exchanges heat, and supplies a liquid refrigerant to the indoor unit and/or the hot water supply unit cooling a heat exchange target with which the liquid refrigerant exchanges heat, and a refrigerant flow control device that controls passage of a refrigerant to the indoor unit and the hot water supply unit; and a control processing device, characterized in that