PURPOSE: To provide a bundle of fine metal bars which can be perfectly controlled in place of a metal small ball, which can not be controlled, as a grinding means of a conventional mechanical parts grinding device.

CONSTITUTION: A grinding means holding means 40 holds the whole of a grinding means 30 and multiple metal bars 31 through the grinding means 30 freely to slide one by one in the vertical direction, and the multiple metal bars 31 are moved vertically together on the material to be ground to hit the material to be ground with a bundle of the fine metal bars. A needle blast device consists of a longitudinal pair of grinding means 30 made of bundle-shape fine metal bars 31, and this pair of grinding means 30 are moved in the vertical direction symmetrically by a seesaw means 54.