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 (31, 34, 35); 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 (31, 34, 35) 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 (31, 34, 35) 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 (31, 34, 35); 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 (31, 34, 35) 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 (31, 34, 35) 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.