PROBLEM TO BE SOLVED: To enable an outer pipe casing to be separated from a ring bit and recovered for certain, after completion of drilling.SOLUTION: An engagement structure between a ring bit and an outer pipe casing has a male screw part formed in a prescribed area at an end close to the outer pipe casing on an outer peripheral surface of the ring bit, and a pin-form protrusion formed on an inner peripheral surface at a distal end side of the outer pipe casing, the pin-form protrusion to be engaged with the male screw part. The ring bit is rotated around an axial center and screwed into an opening on the distal end of the outer pipe casing, to have the pin-form protrusion of the outer pipe casing protrude beyond the male screw part and to the smooth outer peripheral surface of the ring bit.SELECTED DRAWING: Figure 1
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