An air conditioning system, comprising: a condenser for condensing a refrigerant; an evaporator for evaporating the refrigerant passed through the condenser; a compressor for compressing the refrigerant, the compressor having a first compressing part for compressing the refrigerant passed through the evaporator and a second compressing part for compressing both of the refrigerant passed through the first compressing part and a refrigerant injected after being branched from the refrigerant flowing from the condenser to the evaporator; a first expansion device for throttling the refrigerant introduced from the condenser; a phase separator for separating the phase of the refrigerant introduced from the first expansion device; a second expansion device for throttling the liquid refrigerant coming from the phase separator and supplying the same to the evaporator; an injection pipe through which a refrigerant flows; an injection valve disposed on the injection pipe; and a control unit for judging whether a liquid refrigerant is included in the injected refrigerant, characterised in that the control unit is configured such that if a rate of change in a discharge temperature of the second compressing part is outside of a preset normal rate of change in the discharge temperature; or a rate of change of an inlet side temperature of the evaporator is outside of a normal rate of change in inlet evaporator temperature when the rate of change in the discharge temperature of the second compressing part is within the preset normal rate of change; or a difference between an indoor temperature and an outdoor temperature of the air conditioning system is less than a set temperature when the rate of change of the inlet side temperature of the evaporator is within the normal rate of change; or a rate of change of current applied to the compressor is out of a preset normal rate of change of current when the difference between the indoor temperature and the outdoor temperature of the air conditioner is greater than the set temperature, the control unit judges that a liquid refrigerant is included in the injected refrigerant and closes the injection valve for a first set time, and if the number of times of being out of the normal operation range exceeds a set number of times, the injection valve is closed for a second set time which is longer than the first set time. a condenser for condensing a refrigerant; an evaporator for evaporating the refrigerant passed through the condenser; a compressor for compressing the refrigerant, the compressor having a first compressing part for compressing the refrigerant passed through the evaporator and a second compressing part for compressing both of the refrigerant passed through the first compressing part and a refrigerant injected after being branched from the refrigerant flowing from the condenser to the evaporator; a first expansion device for throttling the refrigerant introduced from the condenser; a phase separator for separating the phase of the refrigerant introduced from the first expansion device; a second expansion device for throttling the liquid refrigerant coming from the phase separator and supplying the same to the evaporator; an injection pipe through which a refrigerant flows; an injection valve disposed on the injection pipe; and a control unit for judging whether a liquid refrigerant is included in the injected refrigerant, characterised in that the control unit is configured such that if a rate of change in a discharge temperature of the second compressing part is outside of a preset normal rate of change in the discharge temperature; or a rate of change of an inlet side temperature of the evaporator is outside of a normal rate of change in inlet evaporator temperature when the rate of change in the discharge temperature of the second compressing part is within the preset normal rate of change; or a difference between an indoor temperature and an outdoor temperature of the air conditioning system is less than a set temperature when the rate of change of the inlet side temperature of the evaporator is within the normal rate of change; or a rate of change of current applied to the compressor is out of a preset normal rate of change of current when the difference between the indoor temperature and the outdoor temperature of the air conditioner is greater than the set temperature, the control unit judges that a liquid refrigerant is included in the injected refrigerant and closes the injection valve for a first set time, and if the number of times of being out of the normal operation range exceeds a set number of times, the injection valve is closed for a second set time which is longer than the first set time.