A combined air-conditioning and hot-water supply system comprising: a refrigeration cycle in which a compressor, an outdoor heat exchanger, air-conditioning expansion means, and an indoor heat exchanger are connected to one another in series by using pipes, in which the compressor, the outdoor heat exchanger, hot-water supply expansion means, and a refrigerant-side passage of a refrigerant-heat medium heat exchanger are connected to one another in series by using pipes, in which the refrigerant-side passage of the refrigerant-heat medium heat exchanger and the hot-water supply expansion means are connected in parallel with a bypass path by using pipes, the bypass path including a bypass solenoid valve and bypassing the refrigerant-heat medium heat exchanger, and in which refrigerant circulates; a hot-water supply load circuit in which a circulation pump, a heat medium passage of the refrigerant-heat medium heat exchanger, and a hot-water-storing tank are connected to one another in series by using pipes, and in which a heat medium circulates, the refrigeration cycle and the hot-water supply load circuit being connected in cascade with each other to perform heat exchange between the refrigerant and the heat medium in the refrigerant-heat medium heat exchanger; and a controller that controls the air-conditioning expansion means, the hot-water supply expansion means, and the bypass solenoid valve, characterized in that a first temperature sensor that detects a refrigerant temperature is disposed on a refrigerant inlet side of the refrigerant-heat medium heat exchanger, and a second temperature sensor that detects a heat medium temperature is disposed on a heat medium inlet side of the refrigerant-heat medium heat exchanger, the controller configured to control the bypass solenoid valve and the hot-water supply expansion means in a defrosting operation, based on the refrigerant temperature on the refrigerant inlet side of the refrigerant-heat medium heat exchanger and the heat medium temperature on the heat medium inlet side of the refrigerant-heat medium heat exchanger, wherein in a case where the heat medium temperature is higher than a first temperature and where a value obtained by subtracting the heat medium temperature from a preset heat medium temperature is lower than a second temperature, the controller calculates a third temperature that is a temperature of the refrigerant at which the heat medium freezes, and if the refrigerant temperature is higher than a value obtained by adding a water freezing margin to the third temperature, the controller closes the bypass solenoid valve and fully opens the hot-water supply expansion means. a refrigeration cycle in which a compressor, an outdoor heat exchanger, air-conditioning expansion means, and an indoor heat exchanger are connected to one another in series by using pipes, in which the compressor, the outdoor heat exchanger, hot-water supply expansion means, and a refrigerant-side passage of a refrigerant-heat medium heat exchanger are connected to one another in series by using pipes, in which the refrigerant-side passage of the refrigerant-heat medium heat exchanger and the hot-water supply expansion means are connected in parallel with a bypass path by using pipes, the bypass path including a bypass solenoid valve and bypassing the refrigerant-heat medium heat exchanger, and in which refrigerant circulates; a hot-water supply load circuit in which a circulation pump, a heat medium passage of the refrigerant-heat medium heat exchanger, and a hot-water-storing tank are connected to one another in series by using pipes, and in which a heat medium circulates, the refrigeration cycle and the hot-water supply load circuit being connected in cascade with each other to perform heat exchange between the refrigerant and the heat medium in the refrigerant-heat medium heat exchanger; and a controller that controls the air-conditioning expansion means, the hot-water supply expansion means, and the bypass solenoid valve, characterized in that a first temperature sensor that detects a refrigerant temperature is disposed on a refrigerant inlet side of the refrigerant-heat medium heat exchanger, and a second temperature sensor that detects a heat medium temperature is disposed on a heat medium inlet side of the refrigerant-heat medium heat exchanger, the controller configured to control the bypass solenoid valve and the hot-water supply expansion means in a defrosting operation, based on the refrigerant temperature on the refrigerant inlet side of the refrigerant-heat medium heat exchanger and the heat medium temperature on the heat medium inlet side of the refrigerant-heat medium heat exchanger, wherein in a case where the heat medium temperature is higher than a first temperature and where a value obtained by subtracting the heat medium temperature from a preset heat medium temperature is lower than a second temperature, the controller calculates a third temperature that is a temperature of the refrigerant at which the heat medium freezes, and if the refrigerant temperature is higher than a value obtained by adding a water freezing margin to the third temperature, the controller closes the bypass solenoid valve and fully opens the hot-water supply expansion means.