4. The speech synthesizing apparatus according to claim 1 , wherein the segment selection unit comprises: an optimum segment search unit that searches for optimum segments based on the target segment environment and a segment environment of the candidate segments; 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; and a decision unit that decides, in a case where, among the optimum segments, there exists a segment having a prosody change amount less than the selection criterion, that re-execution of search for an optimum segment is necessary; wherein in a case where the decision unit decides that the re-execution of the search for an optimum segment is necessary, the optimum segment search unit re-executes the search for an optimum segment.
4. The speech synthesizing apparatus according to claim 1 , wherein the segment selection unit comprises: an optimum segment search unit that searches for optimum segments based on the target segment environment and a segment environment of the candidate segments; 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; and a decision unit that decides, in a case where, among the optimum segments, there exists a segment having a prosody change amount less than the selection criterion, that re-execution of search for an optimum segment is necessary; wherein in a case where the decision unit decides that the re-execution of the search for an optimum segment is necessary, the optimum segment search unit re-executes the search for an optimum segment. 6. The speech synthesizing apparatus according to claim 4 , wherein the optimum segment search unit excludes segments that do not satisfy the selection criterion from candidates, and re-executes search for optimum segments.