1. An apparatus for determining, based on speech waveform data, a portion representing a feature of the speech waveform, comprising: an acoustic/prosodic analysis unit which calculates, from said data, a distribution of energy of a prescribed frequency range of said speech waveform along a time axis, and extracts, among various syllables, a first portion of said speech waveform that is generated stably by a source of said speech waveform, based on the distribution of energy and pitch of said speech waveform; a cepstral analysis unit which calculates, from said data, a frequency spectrum distribution of said speech waveform along the time axis, and estimates, based on the frequency spectrum distribution, a second portion of said speech waveform, for which change is well controlled by said source; and a pseudo-syllabic center extracting unit which determines the portion representing the feature of said speech waveform based on the first portion extracted by the acoustic/prosodic analysis unit and the second portion estimated by the cepstral analysis unit, wherein said cepstral analysis unit includes: a linear prediction analysis unit which performs linear prediction analysis on said speech waveform and outputting an estimated value of formant frequency; a cepstral distance calculating unit which calculates, using said data, a distribution of cepstral distance on the time axis based on the estimated value of formant frequency provided by said linear prediction analysis unit; an inter-frame variance calculating unit which calculates, based on an output from said linear prediction analysis unit, distribution of local variance of magnitude of delta cepstrum of said speech waveform on the time axis; and a reliability center candidate output unit which estimates, based both on said distribution of cepstral distance on the time axis based on the estimated value of formant frequency calculated by said cepstral distance calculating unit and on said distribution on the time axis of local variance of magnitude of delta cepstrum of said speech waveform calculated by said inter-frame variance calculating unit, a range in which change in the speech waveform is well controlled by said source.