PURPOSE: To automatically even the height of the sludge inside of a storing pit by providing a means for stopping the operation of a sludge moving means in the case where the maximum value of sludge height detected values becomes nearly equal to the mean value.

CONSTITUTION: A sludge height sensor 11, which is fitted to the upper part of a bucket 4 so as to be projected in the lateral direction, sends the ultrasonic wave toward a storing pit 1 and catches the ultrasonic wave reflected by the sludge 2. The storing pit 1 is divided into plural blocks previously, and the sensor 11 detects the height of the sludge 2 of these blocks. A control device 8 has a memory for storing the sludge height detected value from the sensor 11 and the block, a height judging unit for judging largeness of the sludge height detected value stored in this memory, and a mean value computer for computing the mean value area of the sludge height detected value stored in the memory. The control device 13 falls the sludge with the bucket 4 at an area, of which sludge height detected value is minimum, and stops the operation when the sludge height detected value arrives the mean value.