1. A speech processing system for processing a speech by a computer, comprising: means for dividing said speech signal into frames; means for converting said speech signal divided into frames to a logarithmic power spectrum; means for transforming said logarithmic power spectrum to mel cepstrum coefficients; means for extracting a long-term spectrum variation component from a sequence of said mel cepstrum coefficients by using a longer delta window than an average phoneme duration of an utterance in said speech signal; and means for determining voiced segment by using said long-term spectrum variation component.