Abstract: In a piston for a piston pump including a stopper and a flange fittedly mounted on one end of the piston rod effecting a reciprocating strokes within a cylinder, a reinforcing ring member is fitted between a suction side of a piston packing and a piston member, so that the piston packing can not be forced out when a high pressure is exerted thereon. Also, friction of the piston member relative to the cylinder, which would lead to wear of such members, can be avoided, and in the event that the packing is worn, the airtightness between the piston member and the cylinder can be maintained by the reinforcing ring.