In a conventional display device comprising a sub-display, the display device is increased in thickness and in the number of components as the number of displays is increased. In the present invention, a dual emission display device is used so that either surface of a display is used as a main display or a sub-display. Accordingly, the display device can be reduced in thickness and in the number of components. Further, mechanical reliability can be enhanced when the invention is applied to a tablet PC, a video camera and the like.