1. An apparatus for determining, based on speech waveform data, a portion reliably representing a feature of the speech waveform, comprising: extracting means for calculating, from said data, distribution of an energy of a prescribed frequency range of said speech waveform on a time axis, and for extracting, among various syllables of said speech waveform, a range that is generated stably by a source of said speech waveform, based on the distribution and pitch of said speech waveform; estimating means for calculating, from said data, distribution of spectrum of said speech waveform on the time axis, and for estimating, based on the spectral distribution on the time axis, a range of said speech waveform of which change is well controlled by said source; and means for determining that range which is extracted by said extracting means as the range generated stably by said source and of which speech waveform is estimated by said estimating means to be well controlled by said source, as a highly reliable portion of said speech waveform.