6. The speech synthesis device according to claim 4 , wherein the utterance speed conversion means operates to convert the voice unit data and/or the speech data so as to represent a speech to be uttered at a speed to be produced at a speed satisfying the conditions specified by the utterance speed data, by eliminating a segment representing a phoneme fragment from voice unit data and/or speech data composing data representing the synthesis speech or by adding a segment representing a phoneme fragment to the voice unit data and/or speech data.
6. The speech synthesis device according to claim 4 , wherein the utterance speed conversion means operates to convert the voice unit data and/or the speech data so as to represent a speech to be uttered at a speed to be produced at a speed satisfying the conditions specified by the utterance speed data, by eliminating a segment representing a phoneme fragment from voice unit data and/or speech data composing data representing the synthesis speech or by adding a segment representing a phoneme fragment to the voice unit data and/or speech data. 7. The speech synthesis device according to claim 6 , wherein the voice unit storage means operates to associate phonetic data representing a reading of voice unit with the voice unit data, and the selection means operates to handle voice unit data which is associated with phonetic data representing a reading matching with the reading of the voice unit composing the sentence, as voice unit whose reading is common with the voice unit.