To provide a bracing device of a punching tool applicable to core drills having various kinds of diameters.

This bracing device is constituted of a device substrate 1 provided with a center through-hole 10 to insert the core drill 100 and a mounting member 3 connected to the device substrate 1. In the device substrate 1, three drill receiving materials 2 abutting on an outer peripheral surface of the core drill 100 are provided so that the mounting positions may be varied at a peripheral edge part of the center through-hole 10. The mounting member 3 is provided with a mounting frame body 30 mounted on a guide rod provided in a drill stand and has a mounting plate material 31 provided so that the mounting position may be varied in relation to the device substrate 1. Each of the drill receiving materials 2 is provided with a rotary wheel 20. A height adjusting bolt 12 having a distal end abutting on the surface of a concrete skeleton is provided in an intermediate part at the mounting position of the drill receiving material 2.