PURPOSE: To provide a method to easily form a drill hole a central position of which is precisely set by a core drill regardless of its diameter and material by using a portable drill device.

CONSTITUTION: A drill hole is formed while guiding a drilling tool 4 by a guide member by fixing the guide member formed as a disc 1 on a surface of a structural member 3 at the time of forming the drill hole on the structural member 3 by using the hollow cylindrical drilling tool 4. The guide member is, for example, fixed on the surface of the structural member 3 by adhesion or by a fastener element 2 driven on the structural member. Positional slippage of the drilling tool 4 is prevented by making the guide member engaged with an inner periphery of the drilling tool at the time of driving the drilling tool 4. A small diametrical drilling tool is guided by an outer periphery of a head part of the fastener element 2 driven on the structural member 3 or by an inner periphery of a template, for example.