An upper unit and lower unit are connected by two hinge portions that allow them to move between open and closed positions and to turn around each other. Undercuts are formed on an edge of the upper or lower unit and extensions are formed on the lower or upper unit not provided with undercuts, to mate with the undercuts of the other unit. For the upper unit to turn around the lower unit, the upper unit must first be raised so as not to touch the extensions. Therefore, the present invention prevents scraping damage of surfaces of the upper and lower units.