A plurality of displayable screens are classified into a plurality of screen groups in such a manner that two or more of the screens belong to each individual screen group. Once a desired one of the screen groups is designated by activation of a shift switch, respective tabs of the two or more screens belonging to the designated screen group are visually displayed on a display device. Then, once a desired one of the thus-displayed tabs is designated by activation of a tab switch, the screen corresponding to the thus-designated tab is displayed on the display device. Even when the tab display has been switched by designating another screen group, the screen corresponding to the last designated tab (i.e., main screen), is prevented from being changed unless another tab is designated.