The present invention addresses the problems of enabling a process of converting voice data playback speed even in a voice data playback device alone. The solution is a voice data playback speed conversion method and a voice data playback speed conversion device, comprising: a step of setting a reference zero cross point from any arbitrary zero cross point; a step of selecting a zero cross point temporally after the reference zero cross point within a first predetermined time range; a step of calculating a reference correlation function in a waveform from the reference zero cross point until a second predetermined time; and a step of calculating a correlation function in a waveform from a plurality of previously selected zero cross points until the second predetermined time, wherein a second reference zero cross point is the zero cross point of the waveform having a correlation function in which a concordance rate of the correlation value between the reference correlation function and the correlation function is the highest value, the difference between the reference zero cross point and the second reference zero cross point is calculated as a basic cycle, and the expansion and contraction of voice data is executed in basic cycle units so as to perform a process of converting the playback speed of the voice data.