PURPOSE: To discharge sedimentary sand without being affected by amounts of incoming soil and sand by a method in which a reversing-preventing mechanism to run a chain when lowering an elevator and an elevator frame are provided to the elevator, and the elevator is lowered when a bucket provided to the elevator frame reaches the sedimentary sand.

CONSTITUTION: A groove-shaped side pillar 6 is fitted into an elevator 2 provided to an arm 1 to form an elevator frame 4 to be guided for elevation. Sprockets 8 and 7 are provided to the upper and lower parts of the frame 4, an endless chain 9 is engaged to the sprockets 8 and 7, and a bucket 10 to be turned together is provided to the sprocket 7. A reversing-preventive mechanism 11 to run th chain 9 as the elevator 2 goes down is also provided. When the arm 1 is lowered and the sprocket 7 reaches sedimentary sand, the arm 1 is stopped, the frame 4 is pushed up, and when the elevator 2 is lowered, the chain 9 runs by the mechanism 11, and the bucket 10 is turned through 180° to scoop up the sand.