PURPOSE: To prevent damage to a blind board by giving an impetus with a spring to the blind board so that the blind board swings to shut an opening formed between a bucket body and a movable bucket member.

CONSTITUTION: A movable bucket member 4 is provided to a position facing a bucket body 3, and a blind board 5 is provided for opening and shutting an opening formed between the bucket body 3 and the movable bucket member 4. The blind board 5 is further given an impetus with a spring so that the opening is shut with the blind board 5. With such device, required swing motion is given to the blind board 5 by relatively small force and damage to the blind board can be prevented thereby.