A refrigeration apparatus comprising: a compression mechanism having a plurality of compression elements and configured so that refrigerant discharged from a first-stage compression element of the plurality of compression elements is sequentially compressed by a second-stage compression element; a heat source-side heat exchanger which functions as a radiator or an evaporator of the refrigerant; a usage-side heat exchanger which functions as an evaporator or a radiator of refrigerant; a switching mechanism for switching between a cooling operation state, in which the refrigerant is circulated through the compression mechanism, the heat source-side heat exchanger, and the usage-side heat exchanger in a stated order; and a heating operation state, in which the refrigerant is circulated through the compression mechanism, the usage-side heat exchanger, and the heat source-side heat exchanger in a stated order; a second-stage injection tube (18c, 18h, 19) for branching off refrigerant whose heat has been radiated in the heat source-side heat exchanger or the usage-side heat exchanger and returning the refrigerant to the second-stage compression element; an intermediate heat exchanger which is provided to an intermediate refrigerant tube for drawing into the second-stage compression element refrigerant discharged from the first-stage compression element and which functions as a cooler of refrigerant discharged from the first-stage compression element and drawn into the second-stage compression element during the cooling operation in which the switching mechanism is in the cooling operation state; and an intermediate heat exchanger bypass tube which is connected to the intermediate refrigerant tube so as to bypass the intermediate heat exchanger and which is used to ensure that the refrigerant discharged from the first-stage compression element and drawn into the second-stage compression element is not cooled by the intermediate heat exchanger during the heating operation in which the switching mechanism is in the heating operation state; wherein injection rate optimization control is performed for controlling the flow rate of the refrigerant returned to the second-stage compression element through the second-stage injection tube, so that the injection ratio, which is the ratio of the flow rate of the refrigerant returned to the second-stage compression element through the second-stage injection tube relative to the flow rate of the refrigerant discharged from the compression mechanism, is greater during the heating operation than during the cooling operation. a compression mechanism having a plurality of compression elements and configured so that refrigerant discharged from a first-stage compression element of the plurality of compression elements is sequentially compressed by a second-stage compression element; a heat source-side heat exchanger which functions as a radiator or an evaporator of the refrigerant; a usage-side heat exchanger which functions as an evaporator or a radiator of refrigerant; a switching mechanism for switching between a cooling operation state, in which the refrigerant is circulated through the compression mechanism, the heat source-side heat exchanger, and the usage-side heat exchanger in a stated order; and a heating operation state, in which the refrigerant is circulated through the compression mechanism, the usage-side heat exchanger, and the heat source-side heat exchanger in a stated order; a second-stage injection tube (18c, 18h, 19) for branching off refrigerant whose heat has been radiated in the heat source-side heat exchanger or the usage-side heat exchanger and returning the refrigerant to the second-stage compression element; an intermediate heat exchanger which is provided to an intermediate refrigerant tube for drawing into the second-stage compression element refrigerant discharged from the first-stage compression element and which functions as a cooler of refrigerant discharged from the first-stage compression element and drawn into the second-stage compression element during the cooling operation in which the switching mechanism is in the cooling operation state; and an intermediate heat exchanger bypass tube which is connected to the intermediate refrigerant tube so as to bypass the intermediate heat exchanger and which is used to ensure that the refrigerant discharged from the first-stage compression element and drawn into the second-stage compression element is not cooled by the intermediate heat exchanger during the heating operation in which the switching mechanism is in the heating operation state; wherein injection rate optimization control is performed for controlling the flow rate of the refrigerant returned to the second-stage compression element through the second-stage injection tube, so that the injection ratio, which is the ratio of the flow rate of the refrigerant returned to the second-stage compression element through the second-stage injection tube relative to the flow rate of the refrigerant discharged from the compression mechanism, is greater during the heating operation than during the cooling operation.