PURPOSE: To cut back the projection quantity adjustment man/day of a cutting edge and the change man/day of bore diameter and equipment cost by equipping a tool unit holding means having the tool units in plural kinds cutting edge and an engage ment part driving means and a tool unit feeding means moving along the driving shaft.

CONSTITUTION: A tool unit 14 is moved with its feeding in the axial direction of a spline shaft 12 while being driven with its rotation and a tool unit holding mechanism 15 is approached to the bearing part 1b at the 1st end of a cylinder head 1, then the arm couple 28, 30 at the right side are moved further as they are because of there being no fear of the interference with the bearing part 1b thereof at a standby position to push with the couple of the positioning arm 28 and clamp arm 30 at the left side, the tool unit 14 is inserted into a cam shaft receiving hole 1a at the large and small diameter sides of the bearing part 1b. The unit 14 is then pulled with the arms 28, 30 at the right side with its shifting by its pushing with the left side arms 28, 30 or holding the outer peripheral groove 14b at the right side with the right side arms 28, 30, the cutting edge 20 of the unit 14 under rotation is moved with its slow feed inside a hole 1a and the hole 1a inner peripheral face is lathe-turned in a high accuracy.