1. A speech synthesizing apparatus for performing speech synthesis according to an inputted phoneme string, comprising: storage means for storing statistical data related to a phoneme duration of each phoneme; determining means for determining speech production time of a phoneme string in a predetermined section; setting means for setting a phoneme duration corresponding to the speech production time of each phoneme constructing the phoneme string, based on the statistical data of each phoneme obtained from said storage means; and generating means for generating a speech waveform by connecting phonemes using the phoneme duration.