To mold a tubular part in which a small-diameter pipe part having an inner surface with a diameter smaller than that of a through-hole of a polygonal pipe body is formed integrally at an end part of the body by plastic working.

A tubular part 1 is formed as follows. First, a tubular raw material having a through hole and a polygonal section is inserted into a die 3 having a molding hole 3a and a section formed in the same polygonal shape. Next, a punch 4, which is integrally provided with a reduced cylindrical part 41 in which an outer circumference thereof is identical to a shape of the molding hole 3a of the die 3 and a trumpet-like tapered surface is arranged at a tip end and a molding shaft part 42 arranged at the center of the reduced cylindrical part 41, is pressure inserted from an end part of the tubular raw material up to an intermediate part position in a lengthwise direction, a small-diameter pipe part 14 having an inner surface whose diameter is smaller than that of the through-hole is formed in a way that the small-diameter pipe part 14 with an inner diameter smaller than that of a through-hole 11a of the polygonal pipe body 11 is formed integrally in a polygonal pipe body 11 and an end part thereof via a tapered part 12.