A heat pump comprising: a refrigerant circuit in which a low-stage compression mechanism, a high-stage compression mechanism, a high-temperature heat exchanger, a high-stage expansion mechanism, a low-stage expansion mechanism, and a low-temperature heat exchanger are sequentially connected together by a refrigerant path, and in which a refrigerant dissipates heat into a high-temperature fluid in the high-temperature heat exchanger and the refrigerant absorbs heat from a low-temperature fluid and evaporates in the low-temperature heat exchanger, thereby performing a refrigeration cycle, an auxiliary heat exchanger which is connected to communicate between the refrigerant path between the low-stage compression mechanism and the high-stage compression mechanism and the refrigerant path between the low-stage expansion mechanism and the high-stage expansion mechanism, and which exchanges heat between the refrigerant of the refrigerant circuit and a heat-source fluid, and a compression mechanism control section which controls operational capacity of the high-stage compression mechanism according to a heating load of the high-temperature heat exchanger and controls operational capacity of the low-stage compression mechanism according to a cooling load of the low-temperature heat exchanger, characterized in that a switching mechanism (51, 52) which is configured to, in the case where the heating load is greater than the cooling load, and both of the auxiliary heat exchanger and the low-temperature heat exchanger function as evaporators, switch between a low-stage suction state in which the refrigerant flowing out of the auxiliary heat exchanger is guided to a suction side of the low-stage compression mechanism when a pressure difference between an evaporating pressure of the auxiliary heat exchanger and an evaporating pressure of the low-temperature heat exchanger is smaller than a predetermined value, or when the evaporating pressure of the auxiliary heat exchanger is lower than or equal to the evaporating pressure of the low-temperature heat exchanger, and a high-stage suction state in which the refrigerant flowing out of the auxiliary heat exchanger is guided to a suction side of the high-stage compression mechanism, when the pressure difference is larger than or equal to the predetermined value and the evaporating pressure of the auxiliary heat exchanger is higher than the evaporating pressure of the low-temperature heat exchanger. a refrigerant circuit in which a low-stage compression mechanism, a high-stage compression mechanism, a high-temperature heat exchanger, a high-stage expansion mechanism, a low-stage expansion mechanism, and a low-temperature heat exchanger are sequentially connected together by a refrigerant path, and in which a refrigerant dissipates heat into a high-temperature fluid in the high-temperature heat exchanger and the refrigerant absorbs heat from a low-temperature fluid and evaporates in the low-temperature heat exchanger, thereby performing a refrigeration cycle, an auxiliary heat exchanger which is connected to communicate between the refrigerant path between the low-stage compression mechanism and the high-stage compression mechanism and the refrigerant path between the low-stage expansion mechanism and the high-stage expansion mechanism, and which exchanges heat between the refrigerant of the refrigerant circuit and a heat-source fluid, and a compression mechanism control section which controls operational capacity of the high-stage compression mechanism according to a heating load of the high-temperature heat exchanger and controls operational capacity of the low-stage compression mechanism according to a cooling load of the low-temperature heat exchanger, characterized in that a switching mechanism (51, 52) which is configured to, in the case where the heating load is greater than the cooling load, and both of the auxiliary heat exchanger and the low-temperature heat exchanger function as evaporators, switch between a low-stage suction state in which the refrigerant flowing out of the auxiliary heat exchanger is guided to a suction side of the low-stage compression mechanism when a pressure difference between an evaporating pressure of the auxiliary heat exchanger and an evaporating pressure of the low-temperature heat exchanger is smaller than a predetermined value, or when the evaporating pressure of the auxiliary heat exchanger is lower than or equal to the evaporating pressure of the low-temperature heat exchanger, and a high-stage suction state in which the refrigerant flowing out of the auxiliary heat exchanger is guided to a suction side of the high-stage compression mechanism, when the pressure difference is larger than or equal to the predetermined value and the evaporating pressure of the auxiliary heat exchanger is higher than the evaporating pressure of the low-temperature heat exchanger.