A heat pump apparatus comprising: a refrigeration cycle that includes a compressor, a condenser, an expansion valve, and an evaporator; a water jacket that is arranged on a shell of the compressor and connected in a middle of a branch path branching in parallel to a main circuit which starts flowing from a hot water storage tank to the condenser and returns to the hot water storage tank from the condenser and branching at a branch located at an inlet side of the condenser and at a branch located at an outlet side of the condenser in the main circuit, and that lets water flowing out from the hot water storage tank pass through the water jacket itself; a regulating valve that is connected in a middle of the branch path between the branch at the inlet side and the water jacket, and regulates, according to a control signal having been input, a water flow quantity, characterized by a first temperature sensor that senses a temperature of the shell of the compressor; a second temperature sensor that is installed upstream of the regulating valve and senses a temperature of water flowing out from the hot water storage tank; and a control apparatus that generates the control signal for controlling the regulating valve, based on the temperature sensed by the first temperature sensor and the temperature sensed by the second temperature sensor, and outputs the control signal having been generated to the regulating valve, wherein if the temperature sensed by the first temperature sensor is smaller than a first set value, and if the temperature sensed by the first temperature sensor is smaller than the temperature sensed by the second temperature sensor, the control apparatus generates the control signal for controlling the regulating valve to open so as to heat the shell of compressor; and if the temperature sensed by the first temperature sensor is larger than a second set value, and if the temperature sensed by the first temperature sensor is larger than the temperature sensed by the second temperature sensor, the control apparatus generates the control signal for controlling the regulating valve to open so as to cool the shell of compressor. a refrigeration cycle that includes a compressor, a condenser, an expansion valve, and an evaporator; a water jacket that is arranged on a shell of the compressor and connected in a middle of a branch path branching in parallel to a main circuit which starts flowing from a hot water storage tank to the condenser and returns to the hot water storage tank from the condenser and branching at a branch located at an inlet side of the condenser and at a branch located at an outlet side of the condenser in the main circuit, and that lets water flowing out from the hot water storage tank pass through the water jacket itself; a regulating valve that is connected in a middle of the branch path between the branch at the inlet side and the water jacket, and regulates, according to a control signal having been input, a water flow quantity, characterized by a first temperature sensor that senses a temperature of the shell of the compressor; a second temperature sensor that is installed upstream of the regulating valve and senses a temperature of water flowing out from the hot water storage tank; and a control apparatus that generates the control signal for controlling the regulating valve, based on the temperature sensed by the first temperature sensor and the temperature sensed by the second temperature sensor, and outputs the control signal having been generated to the regulating valve, wherein if the temperature sensed by the first temperature sensor is smaller than a first set value, and if the temperature sensed by the first temperature sensor is smaller than the temperature sensed by the second temperature sensor, the control apparatus generates the control signal for controlling the regulating valve to open so as to heat the shell of compressor; and if the temperature sensed by the first temperature sensor is larger than a second set value, and if the temperature sensed by the first temperature sensor is larger than the temperature sensed by the second temperature sensor, the control apparatus generates the control signal for controlling the regulating valve to open so as to cool the shell of compressor.