PURPOSE: To improve assembly workability and piston sealability by making constitution by dividing a piston rod into one portion containing a rack portion and the other portion, joining them through friction pressure joining and at the same time forming a piston portion integrally with the piston rod by cutting burrs produced at a pressure joining portion.

CONSTITUTION: A piston rod 2 is coaxially divided into one portion 2a containing a rack portion 2c and the other portion 2b, and the portion 2a containing the rack portion 2c is fixed by means of a rotation side chuck 10 and the other portion 2b is fixed by means of a thrust side chuck 11. Jigs 10a, 11a whose purpose is to produce burrs in a vertical direction in regard to the piston rod 2, are provided at chucks 10, 11. And, the portion 2a containing the rack portion 2c is rotated by means of the rotation chuck 10, and the other portion 2b is pressed against the end surface of the portion 2a by means of the thrust side chuck 11, and these are joined. Afterwards, burrs 12 produced at a joining portion are cut, and a piston 3 is formed.