A heat pump apparatus, comprising: a compressor configured to compress refrigerant; a heat exchanger configured to exchange heat between the refrigerant compressed by the compressor and a liquid; a pump configured to pump the liquid; a tank; a controller configured to control heat accumulating operation in which the liquid heated in the heat exchanger is accumulated in the tank; and means for detecting compressor temperature, the compressor temperature being temperature of the compressor or temperature of the refrigerant discharged from the compressor, a temperature sensor for detecting heat pump outlet temperature, the heat pump outlet temperature being temperature of the liquid flowing out of the heat exchanger, characterized in that a start with the compressor temperature at start of the heat accumulating operation higher than a threshold is a hot start, wherein a start with the compressor temperature at start of the heat accumulating operation lower than the threshold is a cold start, wherein the controller is configured to control, in a case of the hot start, a flow rate of the liquid using the pump from a beginning of the heat accumulating operation so that the heat pump outlet temperature matches a target liquid temperature, wherein the controller is configured to divide, in a case of the cold start, the heat accumulating operation into a first stage and a second stage, the second stage being after the first stage, wherein the controller is configured to control, in the first stage of the heat accumulating operation with the cold start, the flow rate of the liquid using the pump so that the heat pump outlet temperature matches a first-stage target temperature that is lower than the target liquid temperature, and wherein the controller is configured to control, in the second stage of the heat accumulating operation with the cold start, the flow rate of the liquid using the pump so that the heat pump outlet temperature matches a second-stage target temperature that is equal to the target liquid temperature. a compressor configured to compress refrigerant; a heat exchanger configured to exchange heat between the refrigerant compressed by the compressor and a liquid; a pump configured to pump the liquid; a tank; a controller configured to control heat accumulating operation in which the liquid heated in the heat exchanger is accumulated in the tank; and means for detecting compressor temperature, the compressor temperature being temperature of the compressor or temperature of the refrigerant discharged from the compressor, a temperature sensor for detecting heat pump outlet temperature, the heat pump outlet temperature being temperature of the liquid flowing out of the heat exchanger, characterized in that a start with the compressor temperature at start of the heat accumulating operation higher than a threshold is a hot start, wherein a start with the compressor temperature at start of the heat accumulating operation lower than the threshold is a cold start, wherein the controller is configured to control, in a case of the hot start, a flow rate of the liquid using the pump from a beginning of the heat accumulating operation so that the heat pump outlet temperature matches a target liquid temperature, wherein the controller is configured to divide, in a case of the cold start, the heat accumulating operation into a first stage and a second stage, the second stage being after the first stage, wherein the controller is configured to control, in the first stage of the heat accumulating operation with the cold start, the flow rate of the liquid using the pump so that the heat pump outlet temperature matches a first-stage target temperature that is lower than the target liquid temperature, and wherein the controller is configured to control, in the second stage of the heat accumulating operation with the cold start, the flow rate of the liquid using the pump so that the heat pump outlet temperature matches a second-stage target temperature that is equal to the target liquid temperature.