PURPOSE: To easily remove dust intruded into a holder, by forming a clearance between the holder and wheel further drilling windows to the holder.

CONSTITUTION: At rolling in a direction Q, a spherical wheel 4 is contacted with the right side part of a side wall 2 to form a large clearance in the left side part of the wall 2, and intruded dust in a holder 1 is discharged from the clearance in the left side in accordance with revolution of the wheel 4. While a change of moving direction of a carrier tool 7 shakes the wheel 4 to change contact and clearance positions repsectively, and the residual dust in the holder 1 is partially discharged from a relocated clearance. Further windows 2a are drilled to the wall 2 of the holder 1, so the dust can be completely removed by blowing compressed air or inserting a brush.