An air conditioning system is able to decrease the amount of refrigerant between a first expansion device and an injection valve and thus adjust the pressure of an injected refrigerant by decreasing the opening degree of the first expansion device and maintaining the opening degree of a second expansion device upon opening of the injection valve, thereby making the system stable upon opening of the injection valve. Furthermore, upon starting up a compressor, the opening degrees of the first and second expansion devices are partly decreased based on the start-up of the compressor and then gradually opened again, and upon completion of the start-up of the compressor, the opening amounts of the first and second expansion devices and the injection valve are controlled, thereby making the cycle more stable.