A method to increase the capacity efficiency of span-restorable mesh networking on sparse facility graphs. When a belt unit is detached from a main body casing, a tension roller is fixed in an outwardly protruding position by a fixing member. The skirt portion of the shadow mask is secured to the inside of the rectangular frame portion, and a bent portion of the shadow mask, which defines a transitional region between the imperforate portion, and the skirt portion is formed in a shape where long sides are recessed inwardly and ends of the long sides of the skirt portion come into contact with the frame portion. The tube connects the valve to the second opening of the tank through the interior chamber of the tank.