An FFT processor applies the fast Fourier transform to each frame of an input signal and generates frequency domain data. A first spectrum calculator calculates the amplitude spectrum of the input signal for the individual frequency components based on the frequency domain data. A dispersion index calculator calculates a dispersion index indicating the level of dispersion of the amplitude spectrum of the input signal in the latest frames. A noise index calculator calculates a noise index indicating the level of noise contained in the input signal. A second spectrum calculator calculates the amplitude spectrum of the noise for the individual frequency components. A gain calculator calculates a spectral gain for suppressing the noise for the individual frequency components. An IFFT processor applies the inverse fast Fourier transform to the frequency domain data after multiplication by the spectral gain at a multiplier, and generates a signal to be output.