PURPOSE: To improve machining efficiency, by providing a drill head with some void from the rotary axis center, while attaching cutting tools in a radial direction and making up a non-cutting zone proximate to said rotary axis center.

CONSTITUTION: A cutting tool 4 is rotated in an arrow direction whereby cutting process goes forward and a core C of a workpiece M develops and grows in a non-cutting zone 5. When the growing core C progressively enters along a cutting tool end face 4b, the partial width of core C is compressed elastically and thereby made to gradually encroach along a tipped end face 6b in consequence, and when pressing force by a tipped part 6 is loaded on the core C as well as when strength (shearing resistance) of the core C succumbs to frictional resistance, the core C is broken off and dropped.