1. A speech synthesis device, comprising: voice unit storage means for storing a plurality of pieces of voice unit data representing voice units; phoneme storage means for storing a plurality of pieces of phoneme data each of which is a phoneme or comprises phoneme fragments composing a phoneme; cadence prediction means for inputting sentence information representing a sentence to predict the cadence of voice units composing the sentence; selecting means using a processor for selecting voice unit data satisfying predetermined conditions out of the plurality of pieces of voice unit data stored in the voice unit storage means, wherein the predetermined conditions are that the voice unit data to be selected matches in its reading with the voice unit composing the sentence and has a correlation greater than a predetermined amount with a cadence prediction result by the cadence prediction means; missing part cadence prediction means using a processor for predicting the cadence of voice units which have been decided not to satisfy the predetermined conditions by the selection means; missing part synthesis means using a processor for specifying phonemes contained in the voice unit decided not to satisfy the predetermined condition by the selection means out of the voice units composing the sentence, for acquiring phoneme data representing the specified phoneme or phoneme fragments composing the specified phoneme from the phoneme storage means, for converting the acquired phoneme data so that the phoneme or phoneme fragments represented by the acquired phoneme data matches with a cadence prediction result by the missing part cadence prediction means, and for interconnecting the converted data, thereby synthesizing speech data representing a waveform of the voice unit; and creation means for interconnecting the voice unit data selected by the selection means and the speech data synthesized by the missing part synthesis means, thereby creating data representing synthesis speech.
8. The speech synthesis device according to claim 1 , 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.