An air conditioner comprising: a compressor configured to compress a refrigerant; a flow passage switching valve configured to switch flow of the refrigerant according to at least one of a cooling mode and a heating mode, respectively; a multistage expansion device configured to decompress the refrigerant, wherein the compressor is a variable speed rotary compressor using the refrigerant as an operating fluid, and the multistage expansion device includes a first expansion valve to decompress a pressure of the refrigerant when the refrigerant is a high-pressure refrigerant and a second expansion valve to decompress the pressure of the refrigerant when the refrigerant is a medium-pressure refrigerant, wherein the first expansion valve and the second expansion valve are connected in series to implement a 2-stage expansion scheme, and adjust a flow rate and a pressure of the refrigerant through change of the degree of opening of the first expansion valve and the degree of opening of the second expansion valve, respectively; a discharge temperature detector configured to detect a refrigerant temperature at an outlet of the compressor; an RPM, revolutions per minute, detector configured to detect RPM of the compressor, an outdoor-temperature detector configured to detect an outdoor temperature; a controller configured to select an initial opening degree of each of the first expansion valve and the second expansion valve in response to the outdoor temperature detected by the outdoor-temperature detector and the compressor RPM detected by the RPM detector; wherein the controller is further configured to control a degree of opening of the first expansion valve and a degree of opening of the second expansion valve, respectively, by detecting the compressor discharge temperature and the compressor RPM at intervals of a predetermined time; wherein the refrigerant is R 32 refrigerant. a compressor configured to compress a refrigerant; a flow passage switching valve configured to switch flow of the refrigerant according to at least one of a cooling mode and a heating mode, respectively; a multistage expansion device configured to decompress the refrigerant, wherein the compressor is a variable speed rotary compressor using the refrigerant as an operating fluid, and the multistage expansion device includes a first expansion valve to decompress a pressure of the refrigerant when the refrigerant is a high-pressure refrigerant and a second expansion valve to decompress the pressure of the refrigerant when the refrigerant is a medium-pressure refrigerant, wherein the first expansion valve and the second expansion valve are connected in series to implement a 2-stage expansion scheme, and adjust a flow rate and a pressure of the refrigerant through change of the degree of opening of the first expansion valve and the degree of opening of the second expansion valve, respectively; a discharge temperature detector configured to detect a refrigerant temperature at an outlet of the compressor; an RPM, revolutions per minute, detector configured to detect RPM of the compressor, an outdoor-temperature detector configured to detect an outdoor temperature; a controller configured to select an initial opening degree of each of the first expansion valve and the second expansion valve in response to the outdoor temperature detected by the outdoor-temperature detector and the compressor RPM detected by the RPM detector; wherein the controller is further configured to control a degree of opening of the first expansion valve and a degree of opening of the second expansion valve, respectively, by detecting the compressor discharge temperature and the compressor RPM at intervals of a predetermined time; wherein the refrigerant is R 32 refrigerant.