An air conditioning system 100 may include a compressor 101 having a driving chamber 111, a heating circuit 310 and a controller 203. This system may release high pressure refrigerant from the compressor discharge port 141 into the compressor driving chamber 111 by opening a capacity control valve 181 when the discharge pressure of the refrigerant discharged from the compressor 101 exceeds a predetermined reference value. By increasing the pressure within the driving chamber 111, the compressor discharge capacity can be reduced. As a result, the discharge pressure of the compressor 101 will be reduced by the reduction in the compressor discharge capacity. Further, the controller 203 may decrease the reference value in accordance with a value related to change in the discharge pressure. As a result, the capacity control valve 181 can be opened at an early stage of the increasing of the discharge pressure if the discharge pressure increases rapidly.