An active noise reducing device is provided with: a control frequency determination unit for determining noise frequency; a reference cosine wave generator for generating reference cosine wave signals having a determined frequency; a reference sine wave generator for generating reference sine wave signals having a noise frequency; a first one-tap adaptive filter to which the reference cosine wave signals are inputted and which has a first filter coefficient; a second one-tap adaptive filter to which the reference sine wave signals are inputted and which has a second filter coefficient; an adder for adding output signals from the first one-tap adaptive filter and output signals from the second one-tap adaptive filter; a secondary noise generator which is driven by the output signals from the adder and which generates secondary noise; a residual sound detector for detecting residual sound generated by interference between the secondary noise and the noise; a simulation signal generator for outputting simulation cosine wave signals and simulation sine wave signals obtained by correcting the reference cosine wave signals and the reference sine wave signals using characteristics resulting from simulating the characteristics of transmission from the secondary noise generator to the residual sound detector; and a filter coefficient updating unit for updating the first filter coefficient and the second filter coefficient on the basis of the output signals from the residual sound detector, the output signals from the simulation signal generator, the reference cosine wave signals, the reference sine wave signals, and the output signals from the adder. The secondary noise reduces noise in the space in which the residual sound detector is installed.