Filtering is performed on original waveform to remove components of a predetermined frequency band from the waveform, and dividing positions of the original waveform data are determined on the basis of envelope levels of the filtered waveform. The dividing positions may be determined on the basis of differentiation of an envelope of the filtered waveform. Rise positions in the original waveform data are detected, and one rise position may be selected from among one or more rise positions detected within a predetermined range of the original waveform and extracted as a dividing position of the original waveform. Presumed beat positions in the original waveform may be detected, and rise positions of the original waveform may be detected within predetermined ranges corresponding to the presumed beat positions. In parallel with reproduction of automatic performance, waveform data are stored in memory along with synchronization control data indicative of relationship in processing timing between the automatic performance and the waveform data.