To provide a heating/cooling system capable of efficiently heating or cooling a heated/cooled room having a large underfloor space.

This heating/cooling system 1 for heating or cooling the heated/cooled room R having the underfloor space S at a back side of a floor material 88, comprises a sectioning material 21 for partitioning the underfloor space S to define an upper space S1 kept into contact with the floor material 88, an underfloor duct 16 for supplying the air AC of a temperature lower than the outside air temperature to the upper space S1 in cooling the heated/cooled room R, and supplying the air AH of a temperature higher than the outside air temperature in heating the heated/cooled room R, and a blower 33 (35) for supplying the air A of a temperature different from the outside air temperature to the underfloor duct 16.