To enable a fast and easy centering of a core drill to be attained with excellent safety.

An indicating piece 12 is integrally arranged at the tip of a shaft 11 to form a centering jig 10 and this is stored in a base 16 of a core drill 14 in such a way that it may be drawn out under a sliding action along an axial direction of the shaft. A drawing-out length of the centering jig 10 from the base 16 set in such a way that a position of the core drill 14 corresponding to a center line 01 of a core bit 18 of the core drill 14 can be indicated by the indicating piece 12 at the drawing-out position.