To prevent the closing on an inner side of a distribution ring by adhesion of sludge powder.

The shape in the cylindrical part of the distribution ring 3A is formed to a truncated cone shape. The powder sludge stuck and deposited on the inside wall of the cylindrical part of the distribution ring 3A is moved to the lower end in the cylindrical part of the frustoconical shape of the distribution ring 3A rotating together with a spindle 2 and is then shaken down to the lower side of the distribution ring 3A. The lower side of a mounting plate 9A for fixing the cylindrical part of the distribution ring 3A at the spindle 2 is also projected out and inclined toward and rotating direction of the spindle 2, by which the sludge powder adhered and deposited to the mounting plate 9A is moved and splashed upward.