1. A method for synthesizing a speech, comprising: dividing a phoneme sequence corresponding to a target speech into a plurality of segments; selecting a plurality of speech units for each segment from a speech unit memory storing speech units having at least one frame, the plurality of speech units having a prosodic feature accordant or similar to the target speech; generating a formant parameter having at least one formant frequency for each frame of the plurality of speech units; corresponding the formant frequencies of the formant parameters among corresponding frames of the plurality of speech units; generating a fused formant parameter of each frame from corresponded formant frequencies of formant parameters of each frame of the plurality of speech units; generating a fused speech unit of each segment from the fused formant parameter of each frame; and generating a synthesized speech by concatenating the fused speech unit of each segment.
14. The method according to claim 1 , wherein selecting comprises estimating a distortion degree between the target speech and the synthesized speech generated using the plurality of speech units; and selecting the plurality of speech units for each segment so that the distortion degree is minimized.