An air conditioner comprising: a compressor, a condenser and an evaporator; a main expansion valve configured to receive refrigerant discharged from the condenser and to throttle the refrigerant; an injection flow path (91, 92) configured to branch off a part of the refrigerant discharged from the main expansion valve as branched refrigerant; an injection expansion valve installed in the injection flow path (91, 92), and configured to receive the branched refrigerant and to throttle the branched refrigerant; an inner heat exchanger configured to: receive the branched refrigerant discharged from the injection expansion valve through the injection flow path (91, 92), receive a main refrigerant, bypassing the injection expansion valve, of the refrigerant discharged from the main expansion valve, exchange heat between the branched refrigerant and the main refrigerant, discharge the main refrigerant to the evaporator, and discharge the branched refrigerant to the compressor through the injection flow path (91, 92); and a controller configured to control the compressor, the main expansion valve and the injection expansion valve; wherein, in an injection cycle of the air conditioner, the controller is configured to: open the injection expansion valve, and adjust an operation frequency of the compressor and/or an opening degree of the main expansion valve, based on state information of the branched refrigerant flowing in the injection flow path (91, 92) through the injection expansion valve; characterized in that the controller is configured to compare a current value of a current applied to the compressor with a predetermined first current value, before the injection expansion valve is open, and wherein the controller is configured to decrease the operation frequency of the compressor, when the current value of the compressor is greater than the first current value a compressor, a condenser and an evaporator; a main expansion valve configured to receive refrigerant discharged from the condenser and to throttle the refrigerant; an injection flow path (91, 92) configured to branch off a part of the refrigerant discharged from the main expansion valve as branched refrigerant; an injection expansion valve installed in the injection flow path (91, 92), and configured to receive the branched refrigerant and to throttle the branched refrigerant; an inner heat exchanger configured to: receive the branched refrigerant discharged from the injection expansion valve through the injection flow path (91, 92), receive a main refrigerant, bypassing the injection expansion valve, of the refrigerant discharged from the main expansion valve, exchange heat between the branched refrigerant and the main refrigerant, discharge the main refrigerant to the evaporator, and discharge the branched refrigerant to the compressor through the injection flow path (91, 92); and a controller configured to control the compressor, the main expansion valve and the injection expansion valve; wherein, in an injection cycle of the air conditioner, the controller is configured to: receive the branched refrigerant discharged from the injection expansion valve through the injection flow path (91, 92), receive a main refrigerant, bypassing the injection expansion valve, of the refrigerant discharged from the main expansion valve, exchange heat between the branched refrigerant and the main refrigerant, discharge the main refrigerant to the evaporator, and discharge the branched refrigerant to the compressor through the injection flow path (91, 92); and a controller configured to control the compressor, the main expansion valve and the injection expansion valve; wherein, in an injection cycle of the air conditioner, the controller is configured to: open the injection expansion valve, and adjust an operation frequency of the compressor and/or an opening degree of the main expansion valve, based on state information of the branched refrigerant flowing in the injection flow path (91, 92) through the injection expansion valve; characterized in that the controller is configured to compare a current value of a current applied to the compressor with a predetermined first current value, before the injection expansion valve is open, and wherein the controller is configured to decrease the operation frequency of the compressor, when the current value of the compressor is greater than the first current value