1. A speech synthesis system for synthesizing speech from text, comprising: a speech segment database for storing data of speech segments having prosody information; means for entering a text to be speech-synthesized; means for determining a speech segment sequence corresponding to the input text from the speech segment database so as to minimize a cost including at least a frequency slope likelihood cost on the basis of a statistical model of prosody variations; means for determining prosody modification values so as to minimize a cost including at least the frequency slope likelihood cost and a prosody modification cost on the basis of the statistical model of prosody variations regarding the determined speech segment sequence; and means for applying the determined prosody modification values to the determined speech segment sequence.