When a processor accepts an input of plural notes to be assigned for tone generation, according to a predetermined note determining rule called “assignment type”, the processor selects one or more notes to be sounded in each of plural parts from the accepted notes. The processor controls a display unit to display, for each of the plural parts, the accepted plural notes to be assigned for tone generation. At this time, non-selected one or more 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 different from the one style. Alternatively, the plural notes to be assigned for tone generation are displayed in one style, and, for each of the plural parts, the selected one or more notes are displayed in another style different from the one style.