PURPOSE: To improve the machining speed by providing rotary motors independently rotating barrel tubs to individually control the grinding pressure and the grinding speed.

CONSTITUTION: The grinding pressures of the media in barrel tubs 21-23 are controlled by the control of the rotating speed of a revolving motor 3, and the grinding speeds are controlled by the control of the rotating speeds of rotating motors 41, 42. The machining efficiency can be sharply improved. The barrel tubs 21-23 are rotated by the independent rotating motors 41, 42 respectively, thus the rotating speeds and rotation directions are optionally controlled, and polishing can be performed while the rotation direction is reversed. The optimum grinding control can be performed in response to the material and quantity of the works and media inserted in the barrel tubs 21-23, and the best machining can be performed.