10. The speech synthesis device according to claim 9 , wherein the executable instructions, when executed by said processor, cause said speech synthesis device to further function as a vocal tract information extraction unit configured to extract vocal tract information from the speech of the speaker, wherein the mouth opening degree calculation unit is configured to calculate a vocal tract cross-sectional area function indicating vocal tract cross-sectional areas, from the vocal tract information extracted by the vocal tract information extraction unit, and calculate, as the mouth opening degree, a sum of the vocal tract cross-sectional areas indicated by the calculated vocal tract cross-sectional area function.
10. The speech synthesis device according to claim 9 , wherein the executable instructions, when executed by said processor, cause said speech synthesis device to further function as a vocal tract information extraction unit configured to extract vocal tract information from the speech of the speaker, wherein the mouth opening degree calculation unit is configured to calculate a vocal tract cross-sectional area function indicating vocal tract cross-sectional areas, from the vocal tract information extracted by the vocal tract information extraction unit, and calculate, as the mouth opening degree, a sum of the vocal tract cross-sectional areas indicated by the calculated vocal tract cross-sectional area function. 11. The speech synthesis device according to claim 10 , wherein the mouth opening degree calculation unit is configured to calculate the vocal tract cross-sectional area function indicating the vocal tract cross-sectional areas on a per section basis, and calculate, as the mouth opening degree, a sum of the vocal tract cross-sectional areas indicated by the calculated vocal tract cross-sectional area function, from a section corresponding to lips up to a predetermined section.