PURPOSE: To provide an assembling method of a piston for a piston pump, integrating a sealing member in the state of no clearance.

CONSTITUTION: A piston body 15 is formed of a body part 15a and a shank 15b protruding from one end of the body part 15a. A backup ring 16 and a sealing member 17 are fitted to the shank 15b in regular succession. A stopper 18 is then fitted to the shank 15b, and the stopper 18 is placed on the sealing member 17 by its own weight so as to be placed in a contact state with the sealing member 17. Finally laser welding is performed to fix the stopper 18 rigidly to the shank 15b. The sealing member 17 is fixed to the shank 15b in the state of no clearance by the contact of the stopper 18 and also in the state of the own weight of the stopper 18 acting thereon.