A phoneme sequence corresponding to a target speech is divided into a plurality of segments. A plurality of speech units for each segment is selected from a speech unit memory that stores speech units having at least one frame. The plurality of speech units has a prosodic feature accordant or similar to the target speech. A formant parameter having at least one formant frequency is generated for each frame of the plurality of speech units. A fused formant parameter of each frame is generated from formant parameters of each frame of the plurality of speech units. A fused speech unit of each segment is generated from the fused formant parameter of each frame. A synthesized speech is generated by concatenating the fused speech unit of each segment.