2. The speech synthesizing apparatus according to claim 1 , wherein the segment selection unit comprises: a prosody change amount calculation unit that calculates a prosody change amount of each candidate segment, based on prosody information of the target segment environment and the candidate segments; a selection criterion calculation unit that calculates a selection criterion, based on the prosody change amount; a candidate selection unit that narrows down selection candidates, based on the prosody change amount and the selection criterion; and an optimum segment search unit that searches for an optimum segment from among the narrowed-down candidate segments; wherein the candidate selection unit excludes, from selection candidates, a segment having a prosody change amount less than the selection criterion, and excludes the segment from a target of search for an optimum segment by the optimum segment search unit.
2. The speech synthesizing apparatus according to claim 1 , wherein the segment selection unit comprises: a prosody change amount calculation unit that calculates a prosody change amount of each candidate segment, based on prosody information of the target segment environment and the candidate segments; a selection criterion calculation unit that calculates a selection criterion, based on the prosody change amount; a candidate selection unit that narrows down selection candidates, based on the prosody change amount and the selection criterion; and an optimum segment search unit that searches for an optimum segment from among the narrowed-down candidate segments; wherein the candidate selection unit excludes, from selection candidates, a segment having a prosody change amount less than the selection criterion, and excludes the segment from a target of search for an optimum segment by the optimum segment search unit. 9. The speech synthesizing apparatus according to claim 2 , wherein the selection criterion calculation unit determines the selection criterion based on an average value of the prosody change amount.