An air conditioner comprising, an exterior heat exchanger (2a, 2b) capable of performing a heat exchange between a refrigerant and an exterior air and including an adjustable valve (4a, 4b) whose opening area is variable so that the adjustable valve is usable as desired one of an expansion valve for the refrigerant to use the exterior heat exchanger as an evaporator and a passage for the refrigerant to use the exterior heat exchanger as a condenser, a first interior heat exchanger capable of performing a heat exchange between the refrigerant and an interior air to be cooled and including another expansion valve for the refrigerant enabling the first interior heat exchanger to be used as another evaporator, a second interior heat exchanger capable of performing as another condenser a heat exchange between the refrigerant and an interior air to be heated, and a compressor for pressurizing the refrigerant, characterized in that a valve means (3a, 3b, 4a, 4b, 5, 8a, 8b, 9a, 9b) is capable of changing a flow course of the refrigerant between: a cooling main course in which the pressurized refrigerant discharged from the compressor flows into each of the second interior heat exchanger and the exterior heat exchanger (2a, 2b) including the adjustable valve used as the passage to use the exterior heat exchanger (2a, 2b) as the condenser, subsequently the condensed refrigerant discharged from each of the second interior heat exchanger and the exterior heat exchanger (2a, 2b) flows into the first interior heat exchanger, and finally the evaporated refrigerant discharged from the first interior heat exchanger returns to the compressor, and a heating main course in which the pressurized refrigerant discharged from the compressor flows into the second interior heat exchanger, subsequently the condensed refrigerant discharged from the second interior heat exchanger flows into each of the first interior heat exchanger and the exterior heat exchanger (2a, 2b) including the adjustable valve used as the expansion valve to use the exterior heat exchanger (2a, 2b) as the evaporator, and finally the evaporated refrigerant discharged from each of the first interior heat exchanger and the exterior heat exchanger (2a, 2b) returns to the compressor; wherein the valve means (3a, 3b, 4a, 4b, 5, 8a, 8b, 9a, 9b) is to change the flow course of the refrigerant from the cooling main course to the heating main course when a temperature of the exterior air is not more than a predetermined temperature, and a difference in absolute value between a desirable amount in absolute value of thermal energy per unit time to be exchanged by the first interior heat exchanger and a desirable amount in absolute value of thermal energy per unit time to be exchanged by the second interior heat exchanger is not more than a predetermined value. a cooling main course in which the pressurized refrigerant discharged from the compressor flows into each of the second interior heat exchanger and the exterior heat exchanger (2a, 2b) including the adjustable valve used as the passage to use the exterior heat exchanger (2a, 2b) as the condenser, subsequently the condensed refrigerant discharged from each of the second interior heat exchanger and the exterior heat exchanger (2a, 2b) flows into the first interior heat exchanger, and finally the evaporated refrigerant discharged from the first interior heat exchanger returns to the compressor, and a heating main course in which the pressurized refrigerant discharged from the compressor flows into the second interior heat exchanger, subsequently the condensed refrigerant discharged from the second interior heat exchanger flows into each of the first interior heat exchanger and the exterior heat exchanger (2a, 2b) including the adjustable valve used as the expansion valve to use the exterior heat exchanger (2a, 2b) as the evaporator, and finally the evaporated refrigerant discharged from each of the first interior heat exchanger and the exterior heat exchanger (2a, 2b) returns to the compressor;