1. A speech synthesis apparatus for synthesizing a speech consisting of a plurality of speech segments each including at least one phoneme, comprising: a database for storing data related to said speech segments; speech segment disassembling means for disassembling each of said speech segments into a plurality of pitch waveforms each having a phase characteristic; phase characteristic transforming means for transforming said phase characteristics of said pitch waveforms into a uniformed phase characteristic for each of said pitch waveforms; pitch waveform classifying means for classifying said pitch waveforms into a plurality of groups each consisting of a plurality of said pitch waveforms substantially identical in shape; pitch waveform registering means for registering said pitch waveforms in said database by extracting one pitch waveform from among said pitch waveforms in each of said groups; and synthesizing means for synthesizing said speech with said pitch waveforms registered in said database.