PURPOSE: To extract a spectrum envelope in consideration of not only the generating process of a speech, but also auditory characteristics.

CONSTITUTION: A power spectrum calculator 15 finds the logarithmic power spectrum of a speech inputted to a speech input device 11 and a masking characteristic calculator 16 finds frequency characteristics of auditory masking from the spectrum. When a vector quantizer 14 quantizes the spectrum envelope, the obtained masking characteristics are compared with the spectrum envelope of the input speech obtained by a cepstrum analyzer 12 and a Fourier feedback unit 13; when the masking characteristics are smaller than the spectrum envelope that a code book indicates in a frequency band wherein the input speech is masked, the weight of the distance scale in the frequency band is set to 0.