1. A method of synthesizing speech comprising: a magnification acquisition step of obtaining, on the basis of target power of synthesized speech, a first magnification to be applied to sub-phoneme unit of a voiced portion and a second magnification to be applied to sub-phoneme units of an unvoiced portions wherein said first magnification is different from said second magnification; an extraction step of extracting sub-phoneme units from a phoneme to be synthesized; an amplitude altering step of altering amplitude of a sub-phoneme unit of a voiced portion of speech waveform, by applying the first magnification to speech waveform of the sub-phoneme, from among the sub-phoneme units extracted at said extraction step, and altering amplitude of a sub-phoneme unit of an unvoiced portion of speech waveform, from among the sub-phoneme units extracted at said extraction step, by applying the second magnification to speech waveform of the sub-phoneme, said amplitude being altered in discrete intervals, and wherein said application of second magnification to the unvoiced portion causes suppression of power of the unvoiced portion; and a synthesizing step of obtaining synthesized speech using the sub-phoneme units processed at said amplitude altering step.