A heat pump apparatus, comprising: a two-cylinder compressor including an electric motor, and two compression units (10, 20) to be driven by the electric motor, the two-cylinder compressor being structured to switch between two operation modes including a single operation in which one of the two compression units (10, 20) is brought into a non-compression state, and a parallel operation in which both the two compression units (10, 20) are brought into a compression state in accordance with operation conditions, a heat-rej ecting-side heat exchanger; a pressure reducing mechanism; a heat-removing-side heat exchanger, the two-cylinder compressor, the heat-rejecting-side heat exchanger, the pressure reducing mechanism, and the heat-removing-side heat exchanger being connected to one another; an inverter drive control device configured to supply drive power to the electric motor of the two-cylinder compressor; characterised by an operation mode detecting-determining unit configured to determine either one of the two operation modes based on an electric signal acquired from the inverter drive control device; and a capacity control device configured to determine rotating frequency of the electric motor based on a result of determination of the operation mode detecting-determining unit so that a temperature of a target object is brought close to a set value, to control the inverter drive control device. a two-cylinder compressor including an electric motor, and two compression units (10, 20) to be driven by the electric motor, the two-cylinder compressor being structured to switch between two operation modes including a single operation in which one of the two compression units (10, 20) is brought into a non-compression state, and a parallel operation in which both the two compression units (10, 20) are brought into a compression state in accordance with operation conditions, an electric motor, and two compression units (10, 20) to be driven by the electric motor, the two-cylinder compressor being structured to switch between two operation modes including a single operation in which one of the two compression units (10, 20) is brought into a non-compression state, and a parallel operation in which both the two compression units (10, 20) are brought into a compression state in accordance with operation conditions, a heat-rej ecting-side heat exchanger; a pressure reducing mechanism; a heat-removing-side heat exchanger, the two-cylinder compressor, the heat-rejecting-side heat exchanger, the pressure reducing mechanism, and the heat-removing-side heat exchanger being connected to one another; the two-cylinder compressor, the heat-rejecting-side heat exchanger, the pressure reducing mechanism, and the heat-removing-side heat exchanger being connected to one another; an inverter drive control device configured to supply drive power to the electric motor of the two-cylinder compressor; characterised by an operation mode detecting-determining unit configured to determine either one of the two operation modes based on an electric signal acquired from the inverter drive control device; and a capacity control device configured to determine rotating frequency of the electric motor based on a result of determination of the operation mode detecting-determining unit so that a temperature of a target object is brought close to a set value, to control the inverter drive control device.