A plurality of template data files are provided, each designating a structure and conditions of a music piece, and having a plurality of tracks, each track being assigned to a particular instrument group and defining a time progression structure of music to be performed by the assigned instrument group by setting performance sections at time positions to be performed by the assigned instrument group along the time progression of music. A plurality of component data files are provided, each representing a length of musical phrase that constitutes a predetermined tone progression pattern of a predetermined tone color for a performance by a particular instrument group. When conditions such as a tempo for a music piece to be created are given, a template data file that satisfies the given conditions is selected. Then, component data files are picked up according to the conditions designated by the selected template data file and the musical phrases are placed on the tracks in the template data file. Thus, data files of many and versatile music pieces are automatically created, satisfying the given conditions.