7. A speech synthesis device comprising: a central segment selection unit for selecting a plurality of central segments from among a plurality of speech segments; a prosody generation unit for generating prosody information for each central segment based on the central segments; a non-central segment selection unit for selecting a non-central segment, which is a segment outside of a central segment section, for each central segment based on the central segments and the prosody information; an optimum central segment selection unit for selecting an optimum central segment from among the plurality of central segments; and a waveform generation unit for generating a synthesized speech waveform based on the optimum central segment, prosody information generated based on an optimum central segment, and a non-central segment selected based on an optimum central segment.
7. A speech synthesis device comprising: a central segment selection unit for selecting a plurality of central segments from among a plurality of speech segments; a prosody generation unit for generating prosody information for each central segment based on the central segments; a non-central segment selection unit for selecting a non-central segment, which is a segment outside of a central segment section, for each central segment based on the central segments and the prosody information; an optimum central segment selection unit for selecting an optimum central segment from among the plurality of central segments; and a waveform generation unit for generating a synthesized speech waveform based on the optimum central segment, prosody information generated based on an optimum central segment, and a non-central segment selected based on an optimum central segment. 9. The speech synthesis device according to claim 7 , wherein the central segment selection unit selects a speech segment from among the plurality of speech segments as a central segment in order of segment length.