PURPOSE: To remove a material from an inner surface without considering the compensation for the deviation from a complete round by moving a cutting means on the inner surface with the relative motion of a cutting tool assembly and a guide means moved along the outer diameter of an article having a closed contour shape.

CONSTITUTION: A cutter 22 is fitted to a fixed plate 24 facing a tappet guide 26, the gap between the cutter 22 and the tappet guide 26 is controlled by a micrometer 28, and the tappet guide 26 is fitted to ride on the outer face 16 of a ring part 10. A spring 30 applies biasing force to a mount 32, and the guide 26 is adjusted along an outer surface 16 in contact with it. If the ring 10 is distorted and its radius is slightly changed, the mount 32 is slid in the radial direction R on an arm 34, and a tool 22 cuts a cover 18 on the portion at a fixed distance from the outer diameter while using the outer diameter as a reference without using the center of the ring 10 as the reference.