Disclosed is a speech synthesis system and method using a smoothing filter. A speech synthesis system for controlling a discontinuous distortion occurred at the transition portion between concatenated phonemes which are speech units of a synthesized speech using a smoothing technique, comprising: a discontinuous distortion processing means adapted to predict a discontinuity occurred at the transition portion between concatenated samples of phonemes used for a speech synthesis through a predetermined learning process, and control a discontinuity occurred at the transition portion between the concatenated phonemes of the synthesized speech in such a fashion that it is smoothed adaptively to correspond to a degree of the predicted discontinuity. The smoothing filter smoothes the synthesized speech so that the discontinuity degree of synthesized speech follows the predicted discontinuity degree according to the filter coefficient (a) changed adaptively to correspond to a ratio of the predicted discontinuity degree to the real discontinuity degree. That is, since a discontinuity occurred at a transition portion between concatenated phonemes of the synthesized speech (IN) is adaptively smoothed to follow that occurred in the actually spoken sound, the synthesized speech (IN) can be approximated more closely to a real human voice.