FIG. 4A indicates a case where Note 1 only is depressed, and the four parts are assigned to Note 1. FIG. 4B indicates a case where Note 2 with a lower pitch than Note 1 is further depressed, wherein Part 1 and Part 2 are assigned to Note 1, and Part 3 and Part 4 are assigned to Note 2. FIG. 4C indicates a case where Note 3 with a lower pitch than Note 2 is further depressed, wherein Part 1 is assigned to Note 1, Part 2 is assigned to Note 2, and Part 3 and Part 4 are assigned to Note 3. FIG. 4D shows a case where Note 4 with a lower pitch than Note 3 is further depressed, wherein Part 1 is assigned to Note 1, Part 2 is assigned to Note 2, Part 3 is assigned to Note 3, and Part 4 is assigned to Note 4. FIG. 4E shows a case where the number of notes is greater than the number of parts, and where Note 5 with a lower pitch than Note 4 is depressed, wherein Part 1 is assigned to Note 1 and Note 2, Part 2 is assigned to Note 3, Part 3 is assigned to Note 4, and Part 4 is assigned to Note 5. In this manner, parts are generally equally assigned to notes.