To easily and precisely perform work for boring a diametrically expanded hole on the periphery of an existing hole by arranging a fitting installing part composed of a taper surface or a columnar surface fitted to and installed in an existing hole port part on the peripheral surface of an auxiliary tool.

An auxiliary tool 1 is made of metal of a disk shape, a shoulder part 3 is arranged in a step shape in a peripheral surface intermediate part, a fitting installing part 2 is arranged in a peripheral surface lower half part, and a holding part 4 is arranged in the center of the auxiliary tool. The fitting installing part 2 is to stabilize the auxiliary tool 1 to a workpiece A by being fitted to and installed in an existing hole port part A' of the workpiece A, and is formed of a taper surface for stabilizing the auxiliary tool 1 by being press-fitted in a tight fit shape over the whole periphery to the existing hole port part A'. The holding part 4 is to hold a drill or a center pin in a loosely inserting shape to the center of a core drill, and the holding part 4 is formed of a through hole bored in the center of the auxiliary tool.