To improve productivity by forming a bar ring part with pressing the neighborhood of prepared hole of preform in the vertical direction and then executing plastic forming to the inner periphery of one end of end part of bar ring part to form a tapered part so as to form an arm in continuous working.

A preform 5, in which a prepared hole of smaller diameter than an inner peripheral hole 8 of an arm 1 is formed beforehand, is held/fixed between a lower die and pressure plate. Next, by descending an upper die, the neighborhood of prepared hole 6 of preform 5 is bent in the direction vertical to the axial line of preform 5 by a punch, the preform 7, in which a bar ring part 3 is formed, is formed. At the state the upper die is stopped, a mandrel of prepared hole and mandrel guide is moved upward. The punch energized with an elastic member is pressed/moved by the mandrel upward, the projected part of mandrel is intruded into the inner peripheral hole 8 of preform 7. The outer peripheral step part of mandrel is pressed to the inner periphery of end part of preform 7 to peripherally form an inclined tapered part 4.