1. A speech synthesis device that generates synthetic speech of text that has been input, the speech synthesis device comprising: a prosody generation unit configured to generate, for each of phonemes generated from the text, a piece of prosody information by using the text; a mouth opening degree generation unit configured to generate, for each of the phonemes generated from the text, a mouth opening degree corresponding to an oral cavity volume, using information generated from the text and indicating a type of the phoneme and a position of the phoneme within the text, the mouth opening degree to be generated being larger for a phoneme positioned at a beginning of a sentence in the text than for a phoneme positioned at an end of the sentence; a segment storage unit in which pieces of segment information are stored, each of the pieces of segment information including a phoneme type, information on a mouth opening degree, and speech segment data; a segment selection unit configured to select, for each of the phonemes generated from the text, a piece of segment information corresponding to the phoneme from among the pieces of segment information stored in the segment storage unit, based on the type of the phoneme and the mouth opening degree generated by the mouth opening degree generation unit; and a synthesis unit configured to generate the synthetic speech of the text, using the pieces of segment information selected by the segment selection unit and the pieces of prosody information generated by the prosody generation unit.