PURPOSE: To appropriately correct a control characteristic while avoiding derivative problems with respect to the change of an acoustic transfer characteristic due to the changing of the opening degree of a damper in an active muffling device for duct.

CONSTITUTION: This device is constituted by providing a secondary sound generator 3 in an air blowing duct 1 in which a damper 2 is interpolatingly arranged and by providing a control means 6 making the secondary sound generator 3 generate a secondary sound made be interferenced in the state of an opposite phase and the same amplitude with respect to a muffling objective sound propagating in the ventilating duct 1. Moreover, the device is provided with an opening degree detecting means 11 detecting the opening degree θ of the damper 2 and a correcting means 10 correcting set characteristics Bs, Cs of the characteristic part X in the control part 6 according to the detected opening degree θof the damper 2 of the opening degree detecting means 11.