PURPOSE: To obtain a sure muffling effect by detecting noise in a blast duct, generating sound wave having the reversed phase and the same sound pressure into the duct from a speaker on the basis of the detected information, and superimposing direct current according to the position of a speaker driving part on an input signal to the speaker.

CONSTITUTION: When a blower 1 is started after starting the operation of an air conditioner, noise in a duct 3 is detected by a sensor microphone 5, and a signal to generate muffling sound wave having the reversed phase and the same sound pressure to the detected noise is formed in a control unit 29, and a sound wave generating signal is output to a speaker unit 11 on the basis of this signal, so as to generate muffling sound wave having the reversed phase and the same sound pressure from a diaphragm 17. When the diaphragm 17 is moved by static pressure, a voice coil 19 directly connected to the diaphragm 17 is also moved in the same direction by the same amount, thereby the moving amount of the voice coil 19 from the vibration center position in the normal state is detected by a position detecting sensor 37, and direct current is transmitted to the speaker unit 11 according to the moving amount, so as to always correct the vibration center position of the diaphragm 17 to the normal position.