Performance information of main music is sequentially acquired, and an accent position of the music is determined. An automatic accompaniment is progressed based on accompaniment pattern data. Upon determination that the current time point coincides with the accent position, an accompaniment event whose tone generation timing arrives within a predetermined time range following the current time point is extracted from the accompaniment pattern data, the tone generation timing of the extracted accompaniment event is shifted to the current time point, and then, accompaniment data is created based on the accompaniment event having the tone generation timing thus shifted. If there is no accompaniment event whose tone generation timing arrives at the current time point or within the predetermined time range following the current time point, automatic accompaniment data with the current time point set as its tone generation timing is additionally created.