To compose a series of musical tones, a waveform generating device reads out a plurality of time axis segments of waveform data in optional order by compressing or expanding the segments of waveform data in conformance with performance data. With the waveform generating device of the present invention, first performance data are updated in conformance with tempo alterations of second performance data. Based on the updating, a time compression and expansion percentage of each waveform data segment is derived. In those cases where there have been performance tempo alterations, the waveform data can be generated in conformance with the tempo alterations.