A heat pump device configured to perform a normal operation for heating water that flows in an water circuit and a defrosting operation that is a reverse cycle of the normal operation by using a refrigerant that circulates, the heat pump device comprising: a control device; a main refrigerant circuit wherein a four-way valve, which is connected to each of a suction port and a discharge port of a compressor by a pipe, and which switches between the normal operation and the defrosting operation by switching a circulation direction of the refrigerant; a heat exchanger that functions as a heat radiator at a time of the normal operation, and that functions as a heat absorber at a time of the defrosting operation; a first decompression device that decompresses the refrigerant that circulates; and an air heat exchanger that functions as the heat absorber at the time of the normal operation and that functions as the heat radiator at the time of the defrosting operation are connected in this order by a pipe, and wherein the refrigerant circulates; and a bypass circuit that connects a discharge side of the compressor, and a connecting part that is a part between the first decompression device and the air heat exchanger, the bypass circuit making a part of a refrigerant that has been discharged from the compressor at the time of the defrosting operation be bypassed as a bypass refrigerant from the main refrigerant circuit to the connecting part; wherein the bypass circuit comprises an electromagnetic valve that switches on and off a bypass of the bypass refrigerant by being controlled by the control device, by being opened and closed, characterized in that said heat exchanger that functions as a heat radiator at a time of the normal operation and as a heat absorber at a time of the defrosting operation is a water heat exchanger that radiates heat to a water at the time of the normal operation and absorbs heat from the water at the time of the defrosting operation, wherein the refrigerant circuit further comprises a bypass refrigerant decompression device that decompresses a bypass refrigerant that has passed the electromagnetic valve, wherein the bypass refrigerant decompression device and the electromagnetic valve are connected and located in a halfway from the discharge side of the compressor to the connecting part; wherein the control device is configured to perform control of opening the electromagnetic valve based on at least one of a water temperature (TW (in)) in a water inlet and a water temperature (TW (out)) in a water outlet of the water heat exchanger at the time of the defrosting operation. a control device; a main refrigerant circuit wherein a four-way valve, which is connected to each of a suction port and a discharge port of a compressor by a pipe, and which switches between the normal operation and the defrosting operation by switching a circulation direction of the refrigerant; a heat exchanger that functions as a heat radiator at a time of the normal operation, and that functions as a heat absorber at a time of the defrosting operation; a first decompression device that decompresses the refrigerant that circulates; and an air heat exchanger that functions as the heat absorber at the time of the normal operation and that functions as the heat radiator at the time of the defrosting operation are connected in this order by a pipe, and wherein the refrigerant circulates; and a bypass circuit that connects a discharge side of the compressor, and a connecting part that is a part between the first decompression device and the air heat exchanger, the bypass circuit making a part of a refrigerant that has been discharged from the compressor at the time of the defrosting operation be bypassed as a bypass refrigerant from the main refrigerant circuit to the connecting part; wherein the bypass circuit comprises an electromagnetic valve that switches on and off a bypass of the bypass refrigerant by being controlled by the control device, by being opened and closed, characterized in that said heat exchanger that functions as a heat radiator at a time of the normal operation and as a heat absorber at a time of the defrosting operation is a water heat exchanger that radiates heat to a water at the time of the normal operation and absorbs heat from the water at the time of the defrosting operation, wherein the refrigerant circuit further comprises a bypass refrigerant decompression device that decompresses a bypass refrigerant that has passed the electromagnetic valve, wherein the bypass refrigerant decompression device and the electromagnetic valve are connected and located in a halfway from the discharge side of the compressor to the connecting part; wherein the control device is configured to perform control of opening the electromagnetic valve based on at least one of a water temperature (TW (in)) in a water inlet and a water temperature (TW (out)) in a water outlet of the water heat exchanger at the time of the defrosting operation.