A heat pump apparatus (100, 200, 300) comprising: - a pressure reducing device configured to expand a heat medium flowing into the pressure reducing device to reduce a pressure of the heat medium, and then allow the heat medium to flow from the pressure reducing device; - a heat-reception-side heat exchanger into which the heat medium flowing from the pressure reducing device flows, the heat-reception-side heat exchanger being configured to cause heat exchange to be performed between the heat medium and a heat source; - a compressor configured to suck the heat medium flowing from the heat-reception-side heat exchanger, compress the heat medium to increase a pressure of the heat medium, and discharge the heat medium; - a heat-storage heat exchanger into which the heat medium discharged from the compressor flows, the heat-storage heat exchanger being configured to cause heat exchange to be performed between the heat medium and a heat storage medium; - a use-side heat exchanger into which the heat medium flows from the heat-storage heat exchanger, the user-side heat exchanger being configured to cause an exchange of heat to be performed between the heat medium and a heat usage medium, and then allow the heat medium to flow from the use-side heat exchanger to the pressure reducing device; and a flow-passage switching device configured to switch a flow passage for the heat medium discharged from the heat-storage heat exchanger to a first flow passage or a second flow passage, the first flow passage extending through the pressure reducing device, the heat-reception-side heat exchanger, the compressor and the heat-storage heat exchanger without extending through the use-side heat exchanger, the second flow passage extending through the pressure reducing device, the heat-reception-side heat exchanger, the compressor and the heat-storage heat exchanger via the use-side heat exchanger, further comprising a first temperature detector configured to detect a first temperature which is a temperature of the heat medium passing through the heat-storage heat exchanger, and a controller configured to control the flow-passage switching device, - wherein the flow-passage switching device switches the flow passage from the first flow passage to the second flow passage, when the first temperature becomes higher than or equal to a flow passage switching temperature set by the controller. - a pressure reducing device configured to expand a heat medium flowing into the pressure reducing device to reduce a pressure of the heat medium, and then allow the heat medium to flow from the pressure reducing device; - a heat-reception-side heat exchanger into which the heat medium flowing from the pressure reducing device flows, the heat-reception-side heat exchanger being configured to cause heat exchange to be performed between the heat medium and a heat source; - a compressor configured to suck the heat medium flowing from the heat-reception-side heat exchanger, compress the heat medium to increase a pressure of the heat medium, and discharge the heat medium; - a heat-storage heat exchanger into which the heat medium discharged from the compressor flows, the heat-storage heat exchanger being configured to cause heat exchange to be performed between the heat medium and a heat storage medium; - a use-side heat exchanger into which the heat medium flows from the heat-storage heat exchanger, the user-side heat exchanger being configured to cause an exchange of heat to be performed between the heat medium and a heat usage medium, and then allow the heat medium to flow from the use-side heat exchanger to the pressure reducing device; and a flow-passage switching device configured to switch a flow passage for the heat medium discharged from the heat-storage heat exchanger to a first flow passage or a second flow passage, the first flow passage extending through the pressure reducing device, the heat-reception-side heat exchanger, the compressor and the heat-storage heat exchanger without extending through the use-side heat exchanger, the second flow passage extending through the pressure reducing device, the heat-reception-side heat exchanger, the compressor and the heat-storage heat exchanger via the use-side heat exchanger, further comprising a first temperature detector configured to detect a first temperature which is a temperature of the heat medium passing through the heat-storage heat exchanger, and a controller configured to control the flow-passage switching device, - wherein the flow-passage switching device switches the flow passage from the first flow passage to the second flow passage, when the first temperature becomes higher than or equal to a flow passage switching temperature set by the controller.