PURPOSE: To prevent a bucket from being buried in sludge by moving a front sprocket wheel and a rear sprocket wheel upward by actuating an oscillating mechanism so as to move the route of a bucket upwardly above a reservoir.

CONSTITUTION: A front sprocket wheel 33 and a rear sprocket wheel 35 are provided at the lower ends of a front swing arm 67 and a rear swing arm 69 which respectively pivot on support axes 63 and 65, while an oscillating mechanism 71 is provided to oscillate the arms 67, 69. Further, one end of a tension arm 85 is connected rotatably to the arm 67, while a free end of a link member 87 which rotates on a support axis 61 is connected to the other end of the arm 85, on which connecting part, a tension-imparting member 89 which is pressed on the outer side of a part of a chain 49 located intermediate between an upper sprocket wheel 41 and the rear sprocket wheel 35 is provided. As a result, a bucket can be surely prevented from being buried in sludge.