A voice synthesis apparatus comprising: a phoneme piece interpolation part that acquires first phoneme piece data of a phoneme piece comprising a sequence of frames and corresponding to a first value of sound characteristic and acquires second phoneme piece data of the phoneme piece comprising a sequence of frames and corresponding to a second value of the sound characteristic different from the first value of the sound characteristic, the first phoneme piece data and the second phoneme piece data indicating a spectrum of each frame of the phoneme piece, wherein the phoneme piece interpolation part interpolates between a spectrum of a frame of the first phoneme piece data and a spectrum of a frame of the second phoneme piece data corresponding to the frame of the first phoneme piece data so as to create phoneme piece data of the phoneme piece corresponding to the target value, in case that both the frame of the first phoneme piece data and the frame of the second phoneme piece data corresponding to the frame of the first phoneme piece data indicate a voiced sound; and a voice synthesis part that generates a voice signal having a target value of the sound characteristic based on the phoneme piece data created by the phoneme piece interpolation part, wherein the phoneme piece interpolation part interpolates by an interpolation rate corresponding to the target value of the sound characteristic which is different from the first value and the second value of the sound characteristic, and wherein the phoneme piece interpolation part interpolates between a sound volume of the frame of the first phoneme piece data and a sound volume of the frame of the second phoneme piece data corresponding to the frame of the first phoneme piece data by the interpolation rate corresponding to the target value of the sound characteristic, and corrects the spectrum of the frame of the first phoneme piece data based on the interpolated sound volume so as to create the phoneme piece data of the target value, in case that either of the frame of the first phoneme piece data or the frame of the second phoneme piece data corresponding to the frame of the first phoneme piece data indicates an unvoiced sound. a phoneme piece interpolation part that acquires first phoneme piece data of a phoneme piece comprising a sequence of frames and corresponding to a first value of sound characteristic and acquires second phoneme piece data of the phoneme piece comprising a sequence of frames and corresponding to a second value of the sound characteristic different from the first value of the sound characteristic, the first phoneme piece data and the second phoneme piece data indicating a spectrum of each frame of the phoneme piece, wherein the phoneme piece interpolation part interpolates between a spectrum of a frame of the first phoneme piece data and a spectrum of a frame of the second phoneme piece data corresponding to the frame of the first phoneme piece data so as to create phoneme piece data of the phoneme piece corresponding to the target value, in case that both the frame of the first phoneme piece data and the frame of the second phoneme piece data corresponding to the frame of the first phoneme piece data indicate a voiced sound; and a voice synthesis part that generates a voice signal having a target value of the sound characteristic based on the phoneme piece data created by the phoneme piece interpolation part, wherein the phoneme piece interpolation part interpolates by an interpolation rate corresponding to the target value of the sound characteristic which is different from the first value and the second value of the sound characteristic, and wherein the phoneme piece interpolation part interpolates between a sound volume of the frame of the first phoneme piece data and a sound volume of the frame of the second phoneme piece data corresponding to the frame of the first phoneme piece data by the interpolation rate corresponding to the target value of the sound characteristic, and corrects the spectrum of the frame of the first phoneme piece data based on the interpolated sound volume so as to create the phoneme piece data of the target value, in case that either of the frame of the first phoneme piece data or the frame of the second phoneme piece data corresponding to the frame of the first phoneme piece data indicates an unvoiced sound.