2. The speech synthesis device according to claim 1 , wherein the executable instructions, when executed by said processor, cause said speech synthesis device to further function as an agreement degree calculation unit configured to, for each of the phonemes generated from the text, select a piece of segment information having a phoneme type that matches the type of the phoneme from among the pieces of segment information stored in the segment storage unit, and calculate a degree of agreement between the mouth opening degree generated by the mouth opening degree generation unit and the mouth opening degree included in the selected piece of segment information, wherein the segment selection unit is configured to select, for each of the phonemes generated from the text, the piece of segment information corresponding to the phoneme, based on the degree of agreement calculated for the phoneme.
2. The speech synthesis device according to claim 1 , wherein the executable instructions, when executed by said processor, cause said speech synthesis device to further function as an agreement degree calculation unit configured to, for each of the phonemes generated from the text, select a piece of segment information having a phoneme type that matches the type of the phoneme from among the pieces of segment information stored in the segment storage unit, and calculate a degree of agreement between the mouth opening degree generated by the mouth opening degree generation unit and the mouth opening degree included in the selected piece of segment information, wherein the segment selection unit is configured to select, for each of the phonemes generated from the text, the piece of segment information corresponding to the phoneme, based on the degree of agreement calculated for the phoneme. 8. The speech synthesis device according to claim 2 , wherein the agreement degree calculation unit is configured to, for each of the phonemes generated from the text, calculate, as the degree of agreement, a degree of agreement between a time direction difference of the mouth opening degree generated by the mouth opening degree generation unit and a time direction difference of the mouth opening degree included in the piece of segment information stored in the segment storage unit and having the phoneme type that matches the type of the phoneme.