To solve the problems that, because the welded part is formed in the tail cap and over the whole circumference of the inside surface of the axis tube, the part is strong in the tensile direction of the axis tube to the tail cap but weak against the force from the lateral direction and that, if the above axis tube is for a writing instrument, since an angle is generated between a paper surface and the instrument, the force acting from the lateral direction of the axis tube becomes large to raise the fear of breakage (separation) of the welded part.

The structure by ultrasonic welding keeps one axis and the other axis joined by ultrasonic welding. Each axis is fitted together, a flange part is formed on at least one of the axis tubes, the end surface of the flange part and the end surface of the other axis are welded, and the inner surface of the one axis and the outer surface of the other axis are welded.