To heighten a supporting strength against the other material to be mounted to a punched portion by making a cutting face of a fracture of the punching portion large when forming the punched portion by punching a given portion of a mother member in its thickness direction.

In the method, when forming the punched portion by punching a given portion Wp of a work W in its board thickness direction, the first step is to form a hardened layer at at least a boundary portion of the given portion and a mother member, wherein the rigidity of the layer is made higher than that of the surrounding portion by a given value. Then, the punching is conducted along the hardened layer. Also, the given portion itself is compressed in its thickness direction to harden the given portion, and the punching is conducted along the boundary of the given portion and the mother member.