Determination is made, in accordance with acquired performance information, as to whether a crossfade characteristic should be changed or not, and a crossfade characteristic of crossfade synthesis having already been started at the time of the acquisition of the performance information was acquired are automatically changed in accordance with a result of the determination. Because the crossfade characteristic is automatically changed during the course of the crossfade synthesis, the time length of the crossfade synthesis can be expanded or contracted as compared to the time length that had been preciously set at the beginning of the crossfade synthesis, and thus, the time position of a succeeding one of rendition style modules to be time-serially combined in accordance with the acquired performance information can be allotted to a time position displaced by an amount corresponding to the expanded or contracted time.