PURPOSE: To make centering secure and easy and stabilize damping force by supporting nearly the middle part of a pipe member having a certain diameter and certain diminsions and forming a piston part by heating and expanding a pipe end part which is to serve as said piston part.

CONSTITUTION: Nearly the middle part of a pipe member having a certain diameter and certain dimensions is supported by a jig and, after providing an oil port 9 and removing burrs, a piston part 5 is heated and machined for expanding, while a screwing part 8 is heated and machined for shrinking the diameter, forming a piston part 5 and a screwing part 8 respectively. Then, each is machined for providing a piston-ring groove and forthreading, etc. respectively. Due to this construction, the center of each part agrees correctly, preventing the dispersion of damping force.