4. The speech synthesis device according to claim 2 , wherein the device further comprises utterance speed conversion means for acquiring utterance speed data specifying conditions of a speed for producing the synthesis speech created by the creation means and for converting the voice unit data and/or speech data so as to represent a speech to be produced at a speed satisfying the conditions specified by the utterance speed data.
4. The speech synthesis device according to claim 2 , wherein the device further comprises utterance speed conversion means for acquiring utterance speed data specifying conditions of a speed for producing the synthesis speech created by the creation means and for converting the voice unit data and/or speech data so as to represent a speech to be produced at a speed satisfying the conditions specified by the utterance speed 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.