1. A method of controlling high-speed reading in a text-to-speech conversion system including a text analysis module for generating a phoneme and prosody character string from an input text; a prosody generation module for generating a synthesis parameter of at least a voice segment, a phoneme duration, and a fundamental frequency for said phoneme and prosody character string; a voice segment dictionary in which voice segments as a source of voice are registered; and a speech generation module for generating a synthetic waveform by waveform superimposition by referring to said voice segment dictionary, said method comprising the step of providing said prosody generation module with a phoneme duration determination unit that includes both a duration rule table containing empirically found phoneme durations and a duration prediction table containing phoneme durations predicted by statistical analysis and determines a phoneme duration by using, when a user-designated utterance speed exceeds a threshold, said duration rule table and, when said threshold is not exceeded, said duration prediction table.