A heat pump water heater apparatus having a water circuit and a refrigerant circuit thermally connected through a first heat exchanger, the refrigerant circuit circulating carbon dioxide, the first heat exchanger being configured to exchange heat between the water and the carbon dioxide, the refrigerant circuit including a compressor, a refrigerant passage of the first heat exchanger, an expansion valve, and a second heat exchanger, the water circuit including a water passage of the first heat exchanger and a tank, the heat pump water heater apparatus comprising: a first sensor configured to detect a temperature of the carbon dioxide discharged from the compressor; a second sensor configured to detect a temperature of the water flowing into the water passage; a third sensor configured to detect a temperature of the water flowing out of the water passage, an accumulator provided in the refrigerant circuit on a downstream side of the second heat exchanger and on an upstream side of the compressor; and a controller, the expansion valve being opened to have an opening degree to reduce a difference between a first value and a target value, the first value being a difference between a temperature detected by the third sensor and a temperature detected by the first sensor, the target value being determined to be smaller in a case where the temperature detected by the second sensor is a first temperature, than in a case of a second temperature, the second temperature being lower than the first temperature, wherein the controller is configured to correct the target value so as to be smaller in a case where the temperature detected by the second sensor is high, than in a case where the temperature detected by the second sensor is low, and characterized in that the refrigerant circuit further includes a liquid level gauge configured to detect a liquid level of the carbon dioxide in the accumulator; and the controller is further configured to prevent the target value from being corrected to be smaller under a state in which the liquid level detected by the liquid level gauge exceeds a threshold value, irrespective of the temperature detected by the second sensor. a first sensor configured to detect a temperature of the carbon dioxide discharged from the compressor; a second sensor configured to detect a temperature of the water flowing into the water passage; a third sensor configured to detect a temperature of the water flowing out of the water passage, an accumulator provided in the refrigerant circuit on a downstream side of the second heat exchanger and on an upstream side of the compressor; and a controller, the expansion valve being opened to have an opening degree to reduce a difference between a first value and a target value, the first value being a difference between a temperature detected by the third sensor and a temperature detected by the first sensor, the target value being determined to be smaller in a case where the temperature detected by the second sensor is a first temperature, than in a case of a second temperature, the second temperature being lower than the first temperature, wherein the controller is configured to correct the target value so as to be smaller in a case where the temperature detected by the second sensor is high, than in a case where the temperature detected by the second sensor is low, and characterized in that the refrigerant circuit further includes a liquid level gauge configured to detect a liquid level of the carbon dioxide in the accumulator; and the controller is further configured to prevent the target value from being corrected to be smaller under a state in which the liquid level detected by the liquid level gauge exceeds a threshold value, irrespective of the temperature detected by the second sensor.