To easily drill a non-through prepared hole even in an insert port having a thin pipe wall in fitting a projection forming ring to the outer periphery of a cut pipe with a small screw.

A stopper 14 projecting the tip of a drill blade 13 by a depth T of the prepared hole 12, the tip face periphery 14a of which is abutted on the peripheral surface 12a of the prepared hole is supported on the tip of the drill blade 13 drilling the prepared hole 12 of a fixing screw 10 of a one-split ring 8 fitted to the outer periphery of the insert opening 3 to form a projecting part 7 in the insert opening 3 so that it cannot be relatively moved in the axial direction to the drill blade 13, but freely rotated around the axis.