A heat pump system comprising: a low temperature heating side refrigerant circuit comprising a low temperature heating side compressor, a low temperature heating side condenser, a low temperature heating side expansion valve and a low temperature heating side evaporator successively connected by a refrigerant pipe; a high temperature heating side refrigerant circuit comprising a high temperature heating side compressor, a high temperature heating side condenser, a high temperature heating side expansion valve and a high temperature heating side evaporator successively connected by a refrigerant pipe; a first pipe (20a, 20b, 20c, 20d, 20e) connecting a low temperature side liquid supply port configured to supply liquid to the system, the low temperature heating side condenser, and the high temperature heating side evaporator in this order, thereby to circulate the liquid; a second pipe (22a, 22b, 22c) connecting a high temperature side liquid supply port configured to supply liquid to the system, and the high temperature heating side condenser in this order, thereby to circulate the liquid; a pump provided in the first pipe (20a, 20b, 20c, 20d, 20e) and configured to feed the liquid heated in the low temperature heating side condenser to the high temperature heating side evaporator; a first three-way valve provided in the first pipe (20a, 20b, 20c, 20d, 20e) between the low temperature heating side condenser and the high temperature heating side evaporator and configured to control a flow rate of the liquid circulated inside the first pipe (20a, 20b, 20c, 20d, 20e); and a control unit configured to control at least one of the pump and the first three-way valve, and control a flow rate of the liquid fed from the low temperature heating side condenser to the high temperature heating side evaporator, characterized in that the heat pump system further comprises: a second three-way valve provided in the second pipe between the high temperature side liquid supply port and the high temperature heating side condenser; and a third pipe configured to connect the first three-way valve and the second three-way valve; wherein the control unit is configured to control the second three-way valve to allow liquid heated in the low temperature heating side condenser to flow into the second three-way valve and to mix with liquid flowing from the high temperature side water supply port to the high temperature heating side condenser. a low temperature heating side refrigerant circuit comprising a low temperature heating side compressor, a low temperature heating side condenser, a low temperature heating side expansion valve and a low temperature heating side evaporator successively connected by a refrigerant pipe; a high temperature heating side refrigerant circuit comprising a high temperature heating side compressor, a high temperature heating side condenser, a high temperature heating side expansion valve and a high temperature heating side evaporator successively connected by a refrigerant pipe; a first pipe (20a, 20b, 20c, 20d, 20e) connecting a low temperature side liquid supply port configured to supply liquid to the system, the low temperature heating side condenser, and the high temperature heating side evaporator in this order, thereby to circulate the liquid; a second pipe (22a, 22b, 22c) connecting a high temperature side liquid supply port configured to supply liquid to the system, and the high temperature heating side condenser in this order, thereby to circulate the liquid; a pump provided in the first pipe (20a, 20b, 20c, 20d, 20e) and configured to feed the liquid heated in the low temperature heating side condenser to the high temperature heating side evaporator; a first three-way valve provided in the first pipe (20a, 20b, 20c, 20d, 20e) between the low temperature heating side condenser and the high temperature heating side evaporator and configured to control a flow rate of the liquid circulated inside the first pipe (20a, 20b, 20c, 20d, 20e); and a control unit configured to control at least one of the pump and the first three-way valve, and control a flow rate of the liquid fed from the low temperature heating side condenser to the high temperature heating side evaporator, characterized in that the heat pump system further comprises: a second three-way valve provided in the second pipe between the high temperature side liquid supply port and the high temperature heating side condenser; and a third pipe configured to connect the first three-way valve and the second three-way valve; wherein the control unit is configured to control the second three-way valve to allow liquid heated in the low temperature heating side condenser to flow into the second three-way valve and to mix with liquid flowing from the high temperature side water supply port to the high temperature heating side condenser. a second three-way valve provided in the second pipe between the high temperature side liquid supply port and the high temperature heating side condenser; and a third pipe configured to connect the first three-way valve and the second three-way valve; wherein the control unit is configured to control the second three-way valve to allow liquid heated in the low temperature heating side condenser to flow into the second three-way valve and to mix with liquid flowing from the high temperature side water supply port to the high temperature heating side condenser.