To provide a waste heat recovering device capable of reducing energy consumption by improving COP in heating operation of an air conditioner by waste heat recovery.

A discharge air side heat exchange part 9 and a supply air side heat exchange part 10 are respectively piping-connected to a compressor 13 at one end of a heat exchanger, and the other end is piping-connected to an expansion valve 14, so that a closed circuit enclosing cooling medium is formed. Cooling medium is compressed by a compressor 13 to be fed to circulate in the closed circuit while repeating phase change to be heat-exchanged. Waste heat is thus recovered.