An expandable tube portion including slots arranged in a staggered fashion is expanded. When the expandable tube portion is expanded, the individual slots open to a cut end portion of the expandable tube portion are transformed to open in V-shape while the whole body of the expandable tube portion is transformed to a lattice configuration. Hence, the tube expansion only need to apply a working force required to bend intersections of the lattice configuration, resulting in the reduction of the working force for tube expansion. As a result, the working force for tube expansion does not open a butt connection portion of a subject matter, negating the need for previously weld bonding the butt connection portion of the subject matter.