1. A prosody modification device comprising: a real voice prosody input part that receives real voice prosody information extracted from an utterance of a human; a modification section determining part that determines a modification section that includes the phoneme or the phoneme string which are to be modified in the real voice prosody information, based on a kind of a phoneme string of the real voice prosody information; a regular prosody generating part that generates regular prosody information having a regular phoneme boundary that determines a boundary between phonemes and a regular phoneme length of a phoneme by using data representing a regular or statistical phoneme length in an utterance of a human with respect to the modification section; and a real voice prosody modification part that resets a real voice phoneme boundary of the phoneme or the phoneme string to be modified in the real voice prosody information by using the regular prosody information generated by the regular prosody generating part so that the real voice phoneme boundary and a real voice phoneme length of the phoneme or the phoneme string to be modified in the real voice prosody information are approximate to an actual phoneme boundary and an actual phoneme length of the utterance of the human, thereby modifying the real voice prosody information.