The present invention discloses an improved air conditioning system which can supply fresh air prepared separately. A first preferred embodiment of the present invention provides an air conditioning system including an outdoor unit, an indoor unit installed in a ceiling, and means connected to the indoor unit for supplying fresh air stored separately to the indoor unit. A second preferred embodiment of the present invention provides an air conditioning system including an outdoor unit, an indoor unit, air supply and air discharge ducts, and means connected to the air supply duct for supplying fresh air stored separately to the indoor unit. In the second embodiment, the indoor unit includes a fan, an indoor heat exchanger, and a guide duct for guiding flow passages of the external air and room air, and, if necessary, further including a preheat exchanger arranged to surround the guide duct for indirect heat exchange between the external air and the indoor air.