PURPOSE: To provide a piston pump/motor excellent in torque performance, rotation stability, low noise, and the like besides facilitating the machining of a notch.

CONSTITUTION: Since the cross-sectional shape of a notch 15 is almost constant, the shape is not changed by polishing, so that appropriate polishing and positioning to the notch 15 are easily compatible, and also the rise of torque is rapid. When a piston comes off a land part, the transmission of pressure from a cocoon-type port 11 is momentarily blocked by a throttle 44 provided in the vicinity of the inlet of a piston hollow part 43, so that the large pressure drop of the high pressure cocoon type port 11 is avoided to eliminate pressure fluctuation.