When a processor receives inputs of plural notes (Nt1, Nt2, Nt3, Nt4), for instance from a keyboard, to be assigned for tone generation, according to a predetermined note determining musical rule called "assignment type" (harmony), the processor autonomously selects (S3), out of the received notes, one or more notes to be sounded in each of plural parts (Parts 1, 2, 3, 4). The processor controls (S4) a display unit (13) to display, for each of the plural parts, the selected plural notes to be assigned for tone generation. Non-selected notes in respective parts are displayed in one style, and the selected one or more notes in the respective parts are displayed in another style (S44).