A hot-water supply system comprising: a compressor for compressing a refrigerant; a first heat exchanger for exchanging heat between the refrigerant compressed by the compressor and a heat medium; a second heat exchanger for exchanging heat between the heat medium flowing out of the first heat exchanger and water; a heat-medium circuit (15, 25, 26, 27, 28, 29) connecting the first heat exchanger to the second heat exchanger; a heat-medium pump for circulating the heat medium in the heat-medium circuit (15, 25, 26, 27, 28, 29); a hot-water storage tank including a lower portion provided with a water outlet, and an upper portion provided with a hot-water inlet at a level higher than a level of the water outlet; a water-feed path connecting the water outlet to the second heat exchanger; a water-return path connecting the second heat exchanger to the hot-water inlet; a water pump for circulating the water in a water circuit formed by the water-feed path, the second heat exchanger, and the water-return path; a heat pump outlet temperature sensor for detecting a heat pump outlet temperature that is a temperature of the heat medium flowing out of the first heat exchanger; an upper tank temperature sensor for detecting an upper tank temperature that is a temperature of water in the upper portion of the hot-water storage tank; and control circuitry (9, 16) configured to control a heat accumulating operation for causing hot water flowing out of the second heat exchanger to flow into the hot-water storage tank, wherein the control circuitry (9, 16) is configured to: in starting the heat accumulating operation, perform a first starting operation and a second starting operation after the first starting operation; in the first starting operation, work the compressor and the heat-medium pump while the water pump is stopped; in the second starting operation, work the compressor, the heat-medium pump, and the water pump; determine a first reference temperature in response to the upper tank temperature detected by the upper tank temperature sensor, the first reference temperature being equal to or higher than the upper tank temperature; and finish the first starting operation and start the second starting operation, when the heat pump outlet temperature reaches the first reference temperature during the first starting operation. a compressor for compressing a refrigerant; a first heat exchanger for exchanging heat between the refrigerant compressed by the compressor and a heat medium; a second heat exchanger for exchanging heat between the heat medium flowing out of the first heat exchanger and water; a heat-medium circuit (15, 25, 26, 27, 28, 29) connecting the first heat exchanger to the second heat exchanger; a heat-medium pump for circulating the heat medium in the heat-medium circuit (15, 25, 26, 27, 28, 29); a hot-water storage tank including a lower portion provided with a water outlet, and an upper portion provided with a hot-water inlet at a level higher than a level of the water outlet; a water-feed path connecting the water outlet to the second heat exchanger; a water-return path connecting the second heat exchanger to the hot-water inlet; a water pump for circulating the water in a water circuit formed by the water-feed path, the second heat exchanger, and the water-return path; a heat pump outlet temperature sensor for detecting a heat pump outlet temperature that is a temperature of the heat medium flowing out of the first heat exchanger; an upper tank temperature sensor for detecting an upper tank temperature that is a temperature of water in the upper portion of the hot-water storage tank; and control circuitry (9, 16) configured to control a heat accumulating operation for causing hot water flowing out of the second heat exchanger to flow into the hot-water storage tank, wherein the control circuitry (9, 16) is configured to: in starting the heat accumulating operation, perform a first starting operation and a second starting operation after the first starting operation; in the first starting operation, work the compressor and the heat-medium pump while the water pump is stopped; in the second starting operation, work the compressor, the heat-medium pump, and the water pump; determine a first reference temperature in response to the upper tank temperature detected by the upper tank temperature sensor, the first reference temperature being equal to or higher than the upper tank temperature; and finish the first starting operation and start the second starting operation, when the heat pump outlet temperature reaches the first reference temperature during the first starting operation. in starting the heat accumulating operation, perform a first starting operation and a second starting operation after the first starting operation; in the first starting operation, work the compressor and the heat-medium pump while the water pump is stopped; in the second starting operation, work the compressor, the heat-medium pump, and the water pump; determine a first reference temperature in response to the upper tank temperature detected by the upper tank temperature sensor, the first reference temperature being equal to or higher than the upper tank temperature; and finish the first starting operation and start the second starting operation, when the heat pump outlet temperature reaches the first reference temperature during the first starting operation.