To provide a pitch change processing technology which causes less damage to the naturalness of voice.

A waveform in the respective pitches of a voice waveform is divided into 1) time α when a minus peak attended with the close of vocal cords appears, 2) a section β where the waveform based on the shape of a vocal tract started from the minus peak appears 3) a section γ where the voice waveform bases on the shape of the vocal tract is converted toward the next minus peak while it is attenuated. Processing of waveform for changing the pitch is executed in a part where the influence of the minus peak attended with the close of the vocal cords is small (namely, the section γ just before the next minus peak) based on the above characteristic of the voice waveform. Thus, the processing can be executed while the shape of the waveform near the peak is completely maintained and the influence of the pitch change is suppressed to be minimum.