PURPOSE: To downsize a space for installation by reducing size from an intake mouth bottom to a floor.

CONSTITUTION: A movable weir is divided and constituted into a movable weir plate 4A, in which a weir plate 4 is mounted horizontally at the upper end of a door body 3, and a pair of left and right vertical fixed weir plates 4B, 4B fastened on the front sides of gate sheets 2 and slid with both end sections in the direction of width of the movable weir plate 4A when the movable weir plate 4A is lifted and lowered. Valve rods 11, 11 lifted and lowered by the opening and closing operation of two switchgears 10, 10 are installed at the two places of both end sections in the direction of width of the door body 3, and the dimension H from an intake mouth bottom 12 to a floor 9 is reduced.