In a hydraulic piston pump, a cylinder port can communicate with a discharge port after a system pressure and a chamber pressure in a cylinder bore becomes in an equilibrium condition. A through hole opening to a surface on which a cylinder block slides in a valve plate is allowed to communicate with a side of one end surface of a balance valve, and the system pressure of a discharge port side is supplied to the other end surface of the balance valve. In the balance valve, a balance piston which slides by a pressure difference between the chamber pressure of the cylinder bore and the system pressure is accommodated. Before the cylinder port communicates with an oil guiding groove, the chamber pressure can be equilibrated to the system pressure by an activation of the balance piston.