A heat pump apparatus, comprising: a refrigerant circuit including a first circuit and a second circuit, the first circuit including a compressor, a refrigerant passage of a use-side heat exchanger, a first pressure reducing device, a first heat-source heat exchanger configured to use outside air serving as a first heat source as a heat source, and a second heat-source heat exchanger; the compressor, the refrigerant passage, the first pressure reducing device, and the first heat-source heat exchanger being sequentially connected, the second circuit including: a second pressure reducing device; and a refrigerant passage of the second heat-source heat exchanger, the refrigerant passage being connected in series to the second pressure reducing device, the second circuit being connected in parallel to the first pressure reducing device and the first heat-source heat exchanger of the first circuit; a heat exchange medium circuit including a heat exchange medium passage of the second heat-source heat exchanger, and configured to circulate therethrough a heat exchange medium serving as a second heat source, the second heat source being used to exchange heat with another heat source other than the outside air so as to receive heat of the another heat source; and a controller configured to switch between a simultaneous operation of causing refrigerant to flow through both of the first heat-source heat exchanger and the second heat-source heat exchanger and a single operation of selecting the first heat-source heat exchanger or the second heat-source heat exchanger to cause refrigerant to flow therethrough, in accordance with a heat exchange amount in each of the first heat-source heat exchanger and the second heat-source heat exchanger. a refrigerant circuit including a first circuit and a second circuit, the first circuit including a compressor, a refrigerant passage of a use-side heat exchanger, a first pressure reducing device, a first heat-source heat exchanger configured to use outside air serving as a first heat source as a heat source, and a second heat-source heat exchanger; the compressor, the refrigerant passage, the first pressure reducing device, and the first heat-source heat exchanger being sequentially connected, the second circuit including: a second pressure reducing device; and a refrigerant passage of the second heat-source heat exchanger, the refrigerant passage being connected in series to the second pressure reducing device, the second circuit being connected in parallel to the first pressure reducing device and the first heat-source heat exchanger of the first circuit; the first circuit including a compressor, a refrigerant passage of a use-side heat exchanger, a first pressure reducing device, a first heat-source heat exchanger configured to use outside air serving as a first heat source as a heat source, and a second heat-source heat exchanger; the compressor, the refrigerant passage, the first pressure reducing device, and the first heat-source heat exchanger being sequentially connected, a compressor, a refrigerant passage of a use-side heat exchanger, a first pressure reducing device, a first heat-source heat exchanger configured to use outside air serving as a first heat source as a heat source, and a second heat-source heat exchanger; the compressor, the refrigerant passage, the first pressure reducing device, and the first heat-source heat exchanger being sequentially connected, the second circuit including: a second pressure reducing device; and a refrigerant passage of the second heat-source heat exchanger, the refrigerant passage being connected in series to the second pressure reducing device, a second pressure reducing device; and a refrigerant passage of the second heat-source heat exchanger, the refrigerant passage being connected in series to the second pressure reducing device, the second circuit being connected in parallel to the first pressure reducing device and the first heat-source heat exchanger of the first circuit; a heat exchange medium circuit including a heat exchange medium passage of the second heat-source heat exchanger, and configured to circulate therethrough a heat exchange medium serving as a second heat source, the second heat source being used to exchange heat with another heat source other than the outside air so as to receive heat of the another heat source; and a controller configured to switch between a simultaneous operation of causing refrigerant to flow through both of the first heat-source heat exchanger and the second heat-source heat exchanger and a single operation of selecting the first heat-source heat exchanger or the second heat-source heat exchanger to cause refrigerant to flow therethrough, in accordance with a heat exchange amount in each of the first heat-source heat exchanger and the second heat-source heat exchanger.