An air-conditioning apparatus comprising: a control unit ; a refrigeration cycle circuit in which a compressor , a first heat exchanger , an expansion device , and a second heat exchanger are connected sequentially to circulate a refrigerant therethrough; a detector ; a first temperature sensor configured to detect a temperature of the refrigerant flowing into the expansion device in a cooling operation in which the second heat exchanger serves as an evaporator; and a third heat exchanger , characterised in that the third heat exchanger is configured to cause the refrigerant flowing through a refrigerant pipe between the first heat exchanger and the expansion device to exchange heat with the refrigerant flowing through a refrigerant pipe between the expansion device and the second heat exchanger , wherein the detector is configured to detect at least one of a temperature and a pressure of the refrigerant flowing through one of the first heat exchanger and the second heat exchanger serving as a condenser, wherein the control unit is configured to control a rotation speed of the compressor , wherein the control unit is configured to, at a time of the cooling operation, control an opening degree of the expansion device based on results of detection by the detector and the first temperature sensor , and configured to control the opening degree of the expansion device such that a difference between a condensing temperature and a detection temperature of the first temperature sensor is within a temperature range, the temperature range being a control target range of the degree of subcooling, the condensing temperature being a condensing temperature of the refrigerant flowing through the condenser, and obtained from a detection value of the detector. a control unit ; a refrigeration cycle circuit in which a compressor , a first heat exchanger , an expansion device , and a second heat exchanger are connected sequentially to circulate a refrigerant therethrough; a detector ; a first temperature sensor configured to detect a temperature of the refrigerant flowing into the expansion device in a cooling operation in which the second heat exchanger serves as an evaporator; and a third heat exchanger , characterised in that the third heat exchanger is configured to cause the refrigerant flowing through a refrigerant pipe between the first heat exchanger and the expansion device to exchange heat with the refrigerant flowing through a refrigerant pipe between the expansion device and the second heat exchanger , wherein the detector is configured to detect at least one of a temperature and a pressure of the refrigerant flowing through one of the first heat exchanger and the second heat exchanger serving as a condenser, wherein the control unit is configured to control a rotation speed of the compressor , wherein the control unit is configured to, at a time of the cooling operation, control an opening degree of the expansion device based on results of detection by the detector and the first temperature sensor , and configured to control the opening degree of the expansion device such that a difference between a condensing temperature and a detection temperature of the first temperature sensor is within a temperature range, the temperature range being a control target range of the degree of subcooling, the condensing temperature being a condensing temperature of the refrigerant flowing through the condenser, and obtained from a detection value of the detector. <EOS>