PROBLEM TO BE SOLVED: To improve water cut-off performance by a simple configuration without complicating a structure in a water gate door.
SOLUTION: An upper step door 14a and a lower step door 14b which approach mutually and leave from each other freely along the direction crossing a pipe end face are provided for an opening 13 provided on the pipe end face of a penstock 11 to open and close the opening 13. A pipe end face seal member 21 for cutting off water by slide-contact of inner faces of the upper step door 14a and the lower step door 14b is provided along a peripheral fringe of the opening 13. A door matching face seal member 23 for cutting off water by close adhesion to the lower step door 14b is provided on a door matching face 22a of the upper step door 14a. A hard plate 31 preventing a side end part 23a from protruding onto a door matching face 22b side of the lower step door 14b is incorporated in a flange part 26 of the door matching face seal member 23.
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