An active noise reducing device comprising: a cosine wave generator for generating a cosine wave signal synchronized with a frequency of noise; a sine wave generator for generating a sine wave signal synchronized with the frequency of the noise; a first one-tap adaptive filter for receiving a reference cosine wave which is an output signal from the cosine wave generator; a second one-tap adaptive filter for receiving a reference sine wave which is an output signal from the sine wave generator; an adder for adding an output signal from the first one-tap adaptive filter to an output signal from the second one-tap adaptive filter; characterized by a plurality of secondary noise generators (28, 29) for generating a secondary noise by using an output signal from the adder; a switcher, disposed between the adder and the plurality of secondary noise generators, for selecting one of the plurality of second noise generators by selectively inputting the adder output signal multiplied by a coefficient to one of the plurality of secondary noise generators; a residual signal detector for detecting a residual signal produced by interference between the noise and the secondary noise which is generated by the secondary noise generator selected by the switcher; and wherein every time a frequency of the noise changes, the switcher is configured to compare gain values with each other, at a present frequency, among a plurality of correction values simulating respective transmission characteristics from the plurality of secondary noise generators to the residual signal detector, and to select the secondary noise generator that makes the value maximum; a simulated signal generator, including a plurality of correction values simulating transmission characteristics from the plurality of the secondary noise generators to the residual signal detector, for outputting a simulated cosine wave signal and a simulated sine wave signal both being corrected with the correction value between the secondary noise generator, which receives the reference cosine wave signal and the reference sine wave signal and is selected by the switcher, and the residual signal detector; and a coefficient updating section for updating respective filter coefficients of the first one-tap adaptive filter and the second one-tap adaptive filter so that noise at the residual signal detector can be minimized by the respective output signals from the residual signal detector and the simulated signal generator. a cosine wave generator for generating a cosine wave signal synchronized with a frequency of noise; a sine wave generator for generating a sine wave signal synchronized with the frequency of the noise; a first one-tap adaptive filter for receiving a reference cosine wave which is an output signal from the cosine wave generator; a second one-tap adaptive filter for receiving a reference sine wave which is an output signal from the sine wave generator; an adder for adding an output signal from the first one-tap adaptive filter to an output signal from the second one-tap adaptive filter; characterized by a plurality of secondary noise generators (28, 29) for generating a secondary noise by using an output signal from the adder; a switcher, disposed between the adder and the plurality of secondary noise generators, for selecting one of the plurality of second noise generators by selectively inputting the adder output signal multiplied by a coefficient to one of the plurality of secondary noise generators; a residual signal detector for detecting a residual signal produced by interference between the noise and the secondary noise which is generated by the secondary noise generator selected by the switcher; and wherein every time a frequency of the noise changes, the switcher is configured to compare gain values with each other, at a present frequency, among a plurality of correction values simulating respective transmission characteristics from the plurality of secondary noise generators to the residual signal detector, and to select the secondary noise generator that makes the value maximum; a simulated signal generator, including a plurality of correction values simulating transmission characteristics from the plurality of the secondary noise generators to the residual signal detector, for outputting a simulated cosine wave signal and a simulated sine wave signal both being corrected with the correction value between the secondary noise generator, which receives the reference cosine wave signal and the reference sine wave signal and is selected by the switcher, and the residual signal detector; and a coefficient updating section for updating respective filter coefficients of the first one-tap adaptive filter and the second one-tap adaptive filter so that noise at the residual signal detector can be minimized by the respective output signals from the residual signal detector and the simulated signal generator.