The speech synthesis apparatus according to the present invention includes a text analyzer operable to generate a phonetic and prosodic symbol string from text information of an input text; a word dictionary storing a reading and accent of a word; a voice segment dictionary storing a phoneme that is a basic unit of speech; a prosody generator operable to generate synthesizing parameters including at least a phoneme, a duration of the phoneme and a fundamental frequency for the phonetic and prosodic symbol string, the prosody generator including a vowel devoicing determining means operable to determine whether or not a vowel devoicing process is to be performed and a duration modifying means operable to modify the duration of the phoneme depending on a speech rate set by a user, the vowel devoicing determining means determining that the vowel devoicing process is not performed when the set speech rate is slower than a predetermined rate; and a waveform generator operable to generate a synthesized waveform by making waveform overlap-adding referring to the synthesizing parameters generated by the prosody generator and the voice segment dictionary.
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