This paper proposes a new approach to speech synthesis based on waveform segments. One novel point of this approach is its new formant frequency modification algorithm which makes it possible to flexibly change formant frequency and so reproduce the desired speech quality. The algorithm characterizes speech formants not only by formant frequencies and formant bandwidths, but also by spectral intensities of formant frequencies. The desirable formant structure, which is specified by the parameters, is obtained by iteratively modifying the formant bandwidths. Using the specified formant structure, the speech signal is synthesized by FFT. Evaluation by the acoustic distance measure, and by listening tests, confirms the good performance of the approach.