1. A system for generating synthetic speech, comprising: a phoneme segment storage section for storing a plurality of phoneme segment data pieces indicating a plurality of sounds of phonemes which are different from each other; and a synthesis section for generating voice data representing synthetic speech of text by receiving an inputted text, by reading out phoneme segment data pieces that correspond to respective phonemes indicating the pronunciation of the inputted text, and then by connecting the read-out phoneme segment data pieces to each other; a computing section for computing a score indicating the unnaturalness of the synthetic speech of the text, on the basis of the voice data; a paraphrase storage section for storing a plurality of second notations, the second notations being paraphrases of first notations and for associating the second notations with the respective first notations; a replacement section for searching the text for a notation matching with any of the first notations and for replacing the searched-out notation with the second notation corresponding to the first notation; and a judgment section for receiving the score and for outputting the generated voice data on condition that the score is smaller than a predetermined reference value, and for inputting the text to the synthesis section in order for the synthesis section to generate further voice data for the text after replacement when the score is equal to or greater than the reference value.