A device is provided with a first die, a second die and a pressing machine. The second die is provided with a plastic processing hole where a core metal is disposed on a hole center line. On the inner peripheral surface of the plastic processing hole, three projecting portions are provided at regular intervals in the circumferential direction of the hole center line. By the approach of the first and second dies to each other, the core metal is inserted into a guide hole of a contact tip, and the projecting portions plastically deform the head end side of the contact tip by pressing it until it is in contact with the core metal.