The position of a connecting portion between waveforms can be arbitrarily set when a plurality of waveform data is obtained by changing pitch widths of stored external sound waveform data, when a read rate of the waveform data is increased or decreased and the readout waveform data is synthesized, or when the waveform is changed from one waveform to another waveform. In addition, a plurality of loop reproduction cycles can be arbitrarily set when the waveform data is to be read out. In loop reproduction, two items of waveform data having different phases between the preset start and end addresses can be repeatedly read out and synthesized, and their mixing ratios can be changed as a function of time, thereby performing loop reproduction so as not to abruptly change amplitude values of the waveforms.