To provide a light-weight piston pump in which power loss of a driving source is low and the slidability is stable.

In this piston pump having a configuration where a plunger 2 is built in a body 6 reciprocatingly to reciprocate by driving force of a driving source 1 for repeating the intake stroke and the discharge stroke while the plunger 2 is held in a non-operating position and detached from the driving source 1, a sleeve 10 is interposed between the body 6 and the plunger 2 for making the body 6 and the plunger 2 relatively movable and a pilot chamber 11 is formed on the outer circumference of the sleeve 10 to move the sleeve 10 by pressure action of the pilot chamber 11 and then to move the plunger 2 to the non-operating position by the moving force of the sleeve 10.