To surely prevent falling-off from a drill holding member of a once installed core drill.
An interposing cylindrical body 40 has a fixed sphere 44 for projecting a part of a peripheral surface toward the cylinder center from an inner peripheral surface, and a movable sphere 45 reciprocally movable in an installing hole 41c of the interposing cylindrical body 40, and projectable toward the cylinder center. While, the drill holding member 20 has a vertical groove 27a recessed on the peripheral surface in response to the fixed sphere 44 and the movable sphere 45, and an engaging groove 27 composed of a first peripheral groove 27b and a second peripheral groove 27c. The engaging groove 27 is formed so that the movable sphere 45 is positioned at the tail end of the second peripheral groove 27c in a state of positioning the fixed sphere 44 at the tail end of the first peripheral groove 27, and is provided with a switching cylindrical body 50 constituted so that a position can be changed between a fitting maintaining position for maintaining a fitting state of the movable sphere 45 to the second peripheral groove 27c and a movement allowing position for allowing the reciprocal movement in the radial direction of the movable sphere 45.
Kyuichi Ueki
Muramatsu Toshiro