To provide an air conditioning system having a simple structure with excellent energy saving property.

An air conditioning system 1 for performing the air conditioning of an air conditioning object space 2 includes: a main return air duct L2 which branches a gas; a total heat exchanger 10 which performs a total heat exchange of the gas; a cooling coil 12 which cools the gas; and a sensible heat exchanger 11 which performs sensible heat exchange of the gas. A return air acquired from the air conditioning object space 2 is branched into a main return air and an exhaust air through the main return air duct L2, and the exhaust air and an outside air acquired from the outside are subjected to total heat exchange in the total heat exchanger 10. The exhaust air is discharged to the outside as an externally exhaust air, while the outside air is mixed, as an externally sucked air, with the main return to form a mixed supply air. The mixed supply air is cooled by a cooling means to form a cooling supply air, the cooling supply air and the main return air are subjected to sensible heat exchange in the sensible heat exchanger 11, and the resulting cooling supply air is supplied, as a supply air, to the air conditioning object space 2.