1. A speech recognition method for recognizing an input speech signal for a plurality of categories, comprising the steps of: storing at least two standard feature vector series having different signal-to-noise ratios (SNRs) for each of said categories; calculating an SNR of said input speech signal; calculating a noise adaptive standard feature vector series by a linear interpolation method between said standard feature vector series using said calculated SNR; and performing a nonlinear expansion and reduction matching operation upon feature vectors of said noise adaptive standard feature vector series and feature vectors of said input speech signal.