To provide a muffler capable of performing precise system identification while driving an engine.

This device is provided with a coefficient operator 43 for forming a muffle signal for generating a sound wave with a size equal to and a phase opposite to a noise from a speaker 6 by an output of a muffle deviation detective microphone 35 and the output of a noise detection microphone 38 through a delay filter 42, an adaptive filter 41 whose coefficient is updated by the coefficient operator, receiving the output of the noise detection microphone and supplying the muffle signal to the speaker and a white noise generator 50. Then, when the system identification is performed before the device is used, the output of the white noise generator is supplied to the adaptive filter and speaker, and the contents of the adaptive filter being an acoustic transmission characteristic on a system path are copied on the delay filter 42. In such a case, a noise source is an internal-combustion engine, and when the system identification is performed, the drive frequency of this internal-combustion engine is made to be changed stepwise or continuously.