PURPOSE: To simultaneously further correctly and promptly perform curved surface machining and finishing of upper surfaces in the main unit part of a metallic belt retainer piece, by using a grinding belt in place of a metallic belt carrier, winding the grinding belt between belt wheels and rotating the belt wheel.

CONSTITUTION: A machining V-belt wheel 18 is secured with a fixed disc 19 to one end part of a shaft 20. A driving surface 21a of a movable disc 21 is placed at a face-to- face position to a driving surface 19a of the disc 19 to form a V-groove, and the groove is arranged with its angle equal to a tapered angle formed by both side surfaces 14a, 14a of the main unit part 14 of a retainer piece 13. While another V-groove is similarly formed by a fixed disc 25 and movable disc 27 of equal shape of the discs 19, 21, and a grinding belt 41 is wound between these V-grooves. When upper surfaces 14c of the main unit part 14 of the retainer piece 13, mounted to the grinding belt 41, is ground, length of a spacer 22 is so adjusted that the radius of curvature r2 of a curve, enveloping the upper surface 14c, may be a desired value, and then a nut 23 is tightened to fix the disc 21 to the disc 19. Then the belt wheel 18 is rotated by a motor 36, and the retainer piece 13 can be ground and finished by grain abrasives 42 of the grinding belt 41.