PURPOSE: To prevent generation of electromagnetic induction by driving a drill by means of an air motor, forming a sucking recess part in the lower surface of a leg part to form a space between the recessed member surface and the lower surface of the leg part, and sucking the air in the space and thereby fixing the leg part to the recessed member surface in the boring device in the headline used for concrete floor surface or the like.

CONSTITUTION: An air motor is used as a rotational driving device 3 of a drill 2. A recess part is formed in a leg part 4 to install a base body 1 on a recessed member surface, and a space part 41 is formed between the leg part and the recessed member surface. The leg part 4 is fixed to the recessed member surface by achieving a suction of the air within this space part to make a vacuum condition by a sucking device. This constitution prevents generation of electromagnetic induction and also adverse effects on D70 exchanging devices or the like.