3. The prosody modification device according to claim 1 , wherein the real voice prosody modification part includes a phoneme boundary resetting part that resets the real voice phoneme boundary of the phoneme or the phoneme string to be modified in the real voice prosody information based on the regular phoneme length of each phoneme of the regular prosody information and a speech rate ratio as a ratio between a rate of speech of the real voice prosody information and a rate of speech of the regular prosody information in the section of the phoneme or the phoneme string to be modified, thereby modifying the real voice prosody information.
3. The prosody modification device according to claim 1 , wherein the real voice prosody modification part includes a phoneme boundary resetting part that resets the real voice phoneme boundary of the phoneme or the phoneme string to be modified in the real voice prosody information based on the regular phoneme length of each phoneme of the regular prosody information and a speech rate ratio as a ratio between a rate of speech of the real voice prosody information and a rate of speech of the regular prosody information in the section of the phoneme or the phoneme string to be modified, thereby modifying the real voice prosody information. 4. The prosody modification device according to claim 3 , further comprising a speech rate ratio detecting part that calculates, in a speech rate calculation range composed of at least one or more phonemes or morae including the phoneme to be modified in the real voice prosody information, the rate of speech of the real voice prosody information for the phoneme to be modified based on a total sum of the real voice phoneme lengths of respective phonemes determined by the real voice phoneme boundary and the number of phonemes or morae in the speech rate calculation range, as well as the rate of speech of the regular prosody information for the phoneme to be modified based on a total sum of the regular phoneme lengths of the respective phonemes determined by the regular phoneme boundary and the number of phonemes or morae in the speech rate calculation range, and calculates the ratio between the rate of speech of the real voice prosody information and the rate of speech of the regular prosody information as the speech rate ratio, wherein the phoneme boundary resetting part calculates a modified phoneme length based on the regular phoneme length of each of the phonemes of the regular prosody information and the speech rate ratio calculated by the speech rate ratio detecting part in the section of the phoneme or the phoneme string to be modified, and resets the real voice phoneme boundary of the real voice prosody information so that each real voice phoneme length in the section becomes the modified phoneme length, thereby modifying the real voice prosody information.