PURPOSE: To facilitate change and assemblage and improve boring accuracy by incorporating a tip in an inclined side end face of the guide part of a holder which is inclined by a predetermined angle to a surface orthogonal to the axis of a reamer.

CONSTITUTION: A tip 1 under the assembled condition makes an angle θ inclination to a surface orthogonal to the axis of a holder 2. The largest outer diameter point 31 of a chamfer is biased to the tip side from the tip position 32 of a second margine 22 by a bearing clearance δ in the axial distance of a reamer. Further, this bearing clearance δ may be of a positive value, preferably a small value. This is because the second margine 22 has also a function of a guide part for ensuring accurate feed of a chamber 3 in the direction of the reamer shaft. When the δ is too large, the working accuracy tends to be degraded.