The present invention provides an air conditioning system including an outdoor unit, an indoor unit, and air supply and air discharge duct. The outdoor unit includes a compressor and an outdoor heat exchanger, and the indoor unit in a ceiling includes an indoor heat exchanger, a fan, guide means, and a preheat exchanger. The indoor heat exchanger has a space therein in communication with a room, and the fan is in the space for drawing air and discharging through the indoor heat exchanger. The guide means is provided on an underside of the indoor heat exchanger, for guiding the external air to the room through the fan, and the room air to an outside of the room. The preheat exchanger is arranged adjacent to, and in communication with the guide means, for indirect heat exchange of the external air and room air passing therethrough. The air supply duct and the air discharge duct are respectively connected to the preheat exchanger, for guiding the external air to the room, and the room air to the outside of the room.