PURPOSE: To correctly form a flat part in the insided of a hole provided to a boiler main body by inserting an edged tool to cut the flat part.

CONSTITUTION: An anchor 1, nut 6 and a tool holder 12, to which an edged tool 11 is set, are mounted to a mandrel 2 in the inside of a main body A. The mandrel 2 is protruded from a hole in the inside of said body A. Then a framework 3 and the whole unit of a device are mounted to the mandrel 2 and fixed by a nut 15. After the tool 11 is positioned to a desired place, a motor is started to perform drilling work. The hole is mostly formed such that a true circle and a long circle are provided neighboring, and the anchor 1 and the holder 12 are placed in the body A for convenience by utilizing the long circle and moved to the true circle.