PURPOSE: To conduct effective grinding of small ball by suspending a holding mechanism of a pipe holding a disc and a small ball at respective gravity center positions so that vibration developed by variation of fricton of the small ball can be delivered to the pipe and the small ball can be rotated at random by the vibration.

CONSTITUTION: Motors 14 and 15 are held by a holding tools 16 and 17 at gravity center position of a pipe-holding mechanism ranging from pipes 6 and 7 to plumbs 18 and 19 attached to the motors 14 and 15. The holders 16 and 17 are suspended from ring-like supporting tables 20 and 21 in such a position that they can be swung in any direction to a certain extent, the pipes 6 and 7 are expanded and contracted while being rotated, and a small ball 24 is ground by being held between side surface of a disc 1 and top ends of the pipes 6 and 7. At this time, vibration induced by variation of friction of the small ball is conducted to the pipes 6 and 7, and thereby whole system of the pipe-holding mechanism is vibrated. This vibration is conversely delivered to the small ball 24, and thereby the small ball 24 is rotated at random to be ground more efficiently.