PURPOSE: To improve the finish by suppressing the temperature rise of a polished metal due to the frictional heat and to improve the polishing efficiency by making the movement of small polishing pieces lively in a device moving the small polishing pieces to polish the metal by utilizing the change of the magnetic field.

CONSTITUTION: A water tank 42 is provided above a rotor 30 rotated by a motor 25, and containers 10 filled with a polished metal and small polishing pieces made of a magnetic raw material are dipped in the water. When permanent magnets 35 are fitted to the rotor 30, magnets 35a of the outer periphery section are made closer to the bottom of the water tank 42 than the magnets 35b of the inner periphery section, more magnets 35a are arranged at the outer periphery section than magnets 35b at the inner periphery section, and the intensity of the magnetic field applied to the water tank 42 is averaged.