An electronic noise attenuation system for attenuation of non-steady noise occurring in a propagation passage such a duct line by conducting adaptive controls by use of a computer system including a digital filter therein. Sensing microphones M.sub.1, M.sub.2 are located in the propagation passage with a cancellation sound source S therebetween at positions where the transfer functions Hr, Ht of a sound wave generated from the cancellation sound source are equivalent to each other. The output signal of the sensing microphone M.sub.1 and the phase-inverted version of the output signal of the sensing microphone M.sub.2 are input via an add circuit to the control part of the system. The system can prevent the occurrence of an acoustic feedback from the cancellation sound source to the sensing microphone M.sub.1 so as to realize a stable and highly accurate noise attenuation.