An audio waveform processing device for performing a frequency analysis on an audio waveform and performing a sine wave or cosine wave synthesis process on each frequency component while performing time compression or expansion and pitch conversion as required, characterized in that: the frequency analysis is performed for each band of an audio signal divided into multiple frequency bands, the audio waveform processing device comprising: reference waveform generating means arranged to generate a reference waveform from an original waveform; comparison means (14, 34) arranged to make a comparison, by using an evaluation function, between a phase condition after the waveform synthesis process of each band and a phase condition of the reference waveform of each band to generate a waveform having a linear phase lead or a linear phase lag with respect to the original waveform highly correlated with the phase condition after the synthesis process as a phase synchronization waveform, wherein a distance on a complex-number plane between the waveforms is used as the evaluation function for evaluating a difference between the phase condition after the waveform synthesis process of each band and the phase condition of the reference waveform of each band; and processing means (15, 35) arranged to perform a cross-fade from the waveform after the synthesis process to the generated phase synchronization waveform in a phase synchronization processing period so that a phase change occurring on the waveform synthesis is reset in the phase synchronization processing period. the frequency analysis is performed for each band of an audio signal divided into multiple frequency bands, the audio waveform processing device comprising: reference waveform generating means arranged to generate a reference waveform from an original waveform; comparison means (14, 34) arranged to make a comparison, by using an evaluation function, between a phase condition after the waveform synthesis process of each band and a phase condition of the reference waveform of each band to generate a waveform having a linear phase lead or a linear phase lag with respect to the original waveform highly correlated with the phase condition after the synthesis process as a phase synchronization waveform, wherein a distance on a complex-number plane between the waveforms is used as the evaluation function for evaluating a difference between the phase condition after the waveform synthesis process of each band and the phase condition of the reference waveform of each band; and processing means (15, 35) arranged to perform a cross-fade from the waveform after the synthesis process to the generated phase synchronization waveform in a phase synchronization processing period so that a phase change occurring on the waveform synthesis is reset in the phase synchronization processing period. reference waveform generating means arranged to generate a reference waveform from an original waveform; comparison means (14, 34) arranged to make a comparison, by using an evaluation function, between a phase condition after the waveform synthesis process of each band and a phase condition of the reference waveform of each band to generate a waveform having a linear phase lead or a linear phase lag with respect to the original waveform highly correlated with the phase condition after the synthesis process as a phase synchronization waveform, wherein a distance on a complex-number plane between the waveforms is used as the evaluation function for evaluating a difference between the phase condition after the waveform synthesis process of each band and the phase condition of the reference waveform of each band; and processing means (15, 35) arranged to perform a cross-fade from the waveform after the synthesis process to the generated phase synchronization waveform in a phase synchronization processing period so that a phase change occurring on the waveform synthesis is reset in the phase synchronization processing period.