To increase a flow rate in the pump chamber of a piston pump so as to improve pump efficiency.

A piston 1 has a plurality of ports P on the circumferential surface thereof, oil is sucked from the ports P in an intake stroke, the ports P are closed during stroke in a compressing stroke, oil is compressed, and then it is supplied to each part. An annular groove f passed the piston stroke direction end rim both sides of the ports P is formed on the inside surface of the piston 1 avoiding the vicinity of the arranging pitch line X-X of the ports P, strength in the vicinity of the arranging pitch line X-X whose strength is lowest is ensured, and the flow rate in a pump chamber is increased, and thereby, pump efficiency is improved.