A carriage for a hard disc drive polished as final finish has high surface precision of micron order. The finish surface is smooth, a cutting trace does not exist, and the surface layer is hardened uniformly by a magnetic material being hit by magnetic power. A cutting bur the edge of which is very tiny does not fall after installing the hard disc drive. The carriage for the magnetic head of the hard disc are not influenced by a temperature change and a seasonal change. The present invention forms a magnetic polishing brush by keeping magnetic abrasive material between magnetic poles in the carriage as a head arm member formed by extrusion molding or drawing for the magnetic head of the hard disc drive. The carriage of non-magnetic material as an aluminum part of HDD inserted in the above described magnetic polishing brush, is given reciprocation motion and vibration, thus the surface of the above described part is polished. The final finish is made by magnetic beam processing to polish or remove burs. Further, surface polish as final finish of the carriage is made by the above described magnetic beam processing using feeble magnetism, the relative magnetic permeability of which is 1.5 to 200, as the above described abrasive material. Further, stainless steel, nickel alloy or iron alloy having the same hardness as said stainless steel is used as the above described polish material.