The present invention relates to a technology capable of providing a hearer with an easy-to-hear synthetic speech to the hearer. The speech synthesizer includes an input unit receiving an input of a sentence, a generation unit generating synthetic speech data from the sentence inputted to the input unit, an accumulation unit accumulating the sentence inputted to the input unit, a collation unit acquiring, when a sentence is newly inputted to the input unit, a collation target sentence that should be collated with this new sentence from the accumulation unit, and calculating a variation degree of the new sentence from the collation target sentence through the collation between the new sentence and the collation target sentence, a calculation unit calculating a variation coefficient corresponding to the variation degree, and a correction unit correcting the synthetic speech data with the variation coefficient.