PURPOSE: To suppress the amount of fluctuation in the cut-in resulting from the rotation of the round tip and impart appropriate rotating force to the round tip, by forming the degree of coaxiality between the inner and outer diameters of the round tip to a predetermined value, and supporting the round tip at a prescribed working face angle.

CONSTITUTION: Since the inner diameter of a shaft-penetration through-hole 5 for a round tip 1 and the outer diameter of the round tip 1 are made to have a coaxiality degree of 3μm or less, the amount of eccentricity of the round tip 1 with respect to a support shaft is suppressed and therefore the fluctuation in the amount of cut-in is suppressed. As a result, the component force applied to the outer periphery of the round tip 1 becomes uniform with the result that the round tip rotates stably. Simultaneously, since the round tip 1 is supported at a working face angle of 16° to 36°, a necessary and sufficient level of driving force is generated, as radial component force needed to stably rotate the round tip 1, with respect to the round tip 1 in which the fluctuation in the cutting resistance resulting from the rotation thereof is suppressed. For this reason, the round tip 1 is reliably rotated and the increase in the rotation speed thereof is suppressed.