Abstract: With a first push rod which is to be inserted/removed into/from a steel pipe through an opening at one end and a second push rod which is to be inserted/removed into/from the steel pipe through an opening at the other end, a metal core disposed on a side on which the first push rod is inserted/removed into/from the die assembly is sandwiched and this metal core is stopped from rotating with the first push rod. With the state of stopping the rotation of the first push rod, the metal core is introduced into the steel pipe through the opening at the one end by both the push rods. After the metal core is pressed into the steel pipe while stopped from rotating with the first push rod, this metal core is pushed back by the second push rod.

Abstract: The one end side portion is nipped by a jig including a jig member having a pair of convex portions which are to be fitted to a depressions in contact with a tooth side face and a inclined portion and positioning a bar and a jig member which is to be clamped/opened with respect to the jig member. A die having an inside diameter smaller than the diameter of the outer periphery is pressed onto the outer periphery of the end portion of the cylindrical end side portion projected from the jig up to a press-in depth determined preliminarily based on the rack, so as to reduce the diameter of the end portion.