1. A speech processing system for processing a speech by a computer, the system comprising: (1) means for dividing an input speech signal into frames; (2) means for converting said input speech signal to a logarithmic power spectrum for each frame; (3) long-term spectrum variation component extraction means comprising: transform means for transforming said logarithmic power spectrum to mel cepstrum coefficients; and extraction means for extracting a long-term spectrum variation component from a sequence of said mel cepstrum coefficients by linear regression calculation using a longer delta window than an average phoneme duration of an utterance in said speech signal, said long-term spectrum variation component comprising a first feature vector for the frame; (4) harmonic structure feature extraction means comprising: discrete cosine transform means for transforming said logarithmic power spectrum to cepstrum coefficients by a discrete cosine transform; clipping means for cutting off upper and lower cepstrum components from said cepstrum coefficients; transform means for inverse discrete cosine transforming said cepstrum coefficients from which said upper and lower cepstrum components have been cut; conversion means for converting an output of said inverse discrete cosine transform back to a power spectrum; processing means for mel filter bank processing said power spectrum; and harmonic structure transform means for transforming a mel filter bank processed output to a harmonic structure feature by said discrete cosine transform, to generate a second feature vector for each frame, the second feature vector comprising the harmonic structure feature; and (5) means for determining a voiced segment by concatenating the first feature vector from said long-term spectrum variation component means and the second feature vector from said harmonic structure feature means and comparing the concatenated feature vectors to a statistical model.