To form a caulking hole for fitting a caulking nut with excellent accuracy.

In a caulking hole forming method for a caulking nut for forming the caulking hole 2 for fitting the caulking nut on a plate 1, the caulking hole 2 includes the same shaped portion 3 connected to a face S1 of one side face of the plate 1 orthogonally to the face S1 and in the same shape, and a tapered portion 4 whose shape becomes gradually larger from the inner edge of the same shaped portion 3 to the other face S2. By bore- driving a punch 5 provided with a boring portion 5a for boring a bore having the same shape as the same shaped portion 3 and the plate 1, and a stepped portion 5b connected to the bored portion 5a and forming the tapered portion 4 by squashing obliquely the inner peripheral edge into the plate 1 placed on the dies 6 having the same center as the bored portion 5a and approximately the same shape as the same shaped portion 3, the caulking hole 2 is formed.