PURPOSE: To manufacture efficiently a product having a good through-hole by making the wall surface of the peripheral part vertical, forming a connecting part of said wall surface and the bottom part to a square shape, providing a hole having a chamfer in the opening end, and subsequently, executing the piercing by a regular method.

CONSTITUTION: A tip part 18 of a mandrel 12 is connected to a base part 17 of a large diameter through a chamfer 19, and the outside periphery of its tip part 18 is formed in parallel to an axis of the mandrel. By such a mandrel 12, a vertical wall surface 41 is formed, and a square part 43 of a small curve is formed in an intersection of the wall surface 41 and the bottom face of a hole 40. In the next piercing process, a stress by a punch 22 is concentrated on the square part 43, and a good pierced part is formed. In such a way, a product having a good through-hole can be manufactured efficiently.