To relatively easily detach a drill bit in an excessively fastened state without damaging the drill bit.

The drill bit 10 is assembled in a assembling end part 36 of a drill string 20 by this fracture type washer 24. This fracture type washer 24 is provided with a ring body having an inside diameter and an outside diameter. A first surface 40 of the ring body is a diametrically extending flat surface and approximately perpendicular to the central axis 14 of the body. A second surface 42 diametrically extends being inclined to the first surface 40 and consequently, this fracture type water 24 becomes a section gradually increasing its thickness in the diametrical direction.