When the on-on time between Note 1 and Note 2 is shorter than the double stop judgment time JT, as shown in FIG. 3A, the mode is changed from Unison 1 to Unison 2. When note-on information of Note 1 is inputted at time t1, the parts 1-4 are assigned to Note 1, and simultaneously start sound generation at pitch n1, as shown in FIG. 3B. Next, when note-on information of Note 2 at pitch n2 lower than Note 1 is inputted at time t2, the mode is switched to Unison 2. Part 1 (with the timbre being trumpet) and Part 2 (with the timbre being clarinet) which are higher in the pitch order are assigned to Note 1, and continue generating the musical sound at pitch n1 of Note 1, and Part 3 (with the timbre being alto saxophone) and Part 4 (with the timbre being trombone) which are lower in the pitch order are assigned to Note 2, stop the sound generation at pitch n1, and start sound generation at pitch n2 of Note 2.