PURPOSE: To increase the colliding force of media and to prevent choking of media in the through-hole of a structure part to be polished by employing triangular pole media where the height is longer than predetermined times of average side of triangular cross-section.

CONSTITUTION: Media 3 has such size and shape as insertable into the through- hole 1 where the height is longer than 2 times of the average side of triangle. Consequently, it will never clog vertically in the axial direction of through-hole 1 and easily drawable because the height of triangular pole is longer than 2 times and shorter than 3 times of the side. In order to achieve highly accurate polishing, a right triangular pole media having approximately triangular upper and lower faces is preferable. Because of high impact force of media, the polishing time is shortened resulting in precise polishing without causing clogging.