To certainly drill a large hole by providing a blade and a return stop on an upper part, making the blade in an angular and bent shape and providing the return stop inside of the bent blade and at a position lower than the lowest part of the blade.

An angular and orthogonally bent blade 2 is provided on an upper part, and a return stop 3 in a shape to make a plate vertical inside of this orthogonally bent blade 2 is provided. An upper end of this return stop 3 is lower than the lowest end 5 of the blade 2, and this return stop 3 is made so that an end part of its outside is projected out of a vertical surface including a line connecting the lowest ends 5, 5 of the blade 2 vertically. Consequently, it is forcibly pushed out to the outside from the line connecting end parts of the cut part on the upper side of the part the periphery of which is cut out, and accordingly, a part lifted up by the return stop 3 is bent and it never returns to its origin.