PURPOSE: To improve productivity of a piston pump by a method wherein a check valve opened by working fluid delivered in a delivery chamber through a delivery port from a cylinder line is mounted on a pump housing through a cylinder liner.

CONSTITUTION: A delivery port 26 is opened in the bottom of a cylinder liner 6 and a check valve 11 to open the delivery port 26 by working oil delivered from an oil pressure chamber 25 is provided. Though each piston 5 is reciprocated over a cylinder line 6 along with rotation of a cam plate 3, at a process wherein a piston 5 is pushed out from the cylinder liner 6, working oil in an intake chamber 10 is sucked through a suction port 9 to the oil pressure chamber 25. Further, at a process wherein the piston 5 is pushed in the cylinder liner 6, working oil boosted by the piston 5 opens the check valve 11 and is delivered to the delivery chamber 12 from the oil pressure chamber 25 to a delivery chamber 12 and delivered from the delivery chamber 12 through a delivery port 14.