PROBLEM TO BE SOLVED: To ensure an easy mounting of a shutter and a smooth and reliable operation. SOLUTION: A cover 14 that forms a lift way of buckets 10 having a rectangular shape in plane view is arranged inside of a soil supply hopper. The cover 14 consists of a front board 15 and left and right side boards 16. A flow-in mouth 17 through which soil flow into the lift way is formed at the bottom of the cover 14 by cutting off a part of the front board 15. The side boards 16 at the left and right sides of the flow-in mouth 17 are bent forward with a designated width kept to thereby form a pair of attachment parts 18. A designated clearance is maintained between the inside surface of the attachment parts 18 and the outside surface of the front board 15, and left and right sides of a shutter 20 formed from a plate with an operation handle 21 attached at the left-right center are fit into the clearance. A longitudinal oblong hole 22 is formed on the left and right attachment parts 18 and a bolt 23 is inserted through the hole 22 and fixed on to the shutter 20.
