To provide an axial piston pump motor, in which the balance of fluid pressure applied to a piston can be easily secured with good accuracy and the working force in the direction different from the reciprocating direction of a piston is little generated in a space up to a cylinder block.

An axial piston pump motor comprises a cylinder block 31 capable of freely rotating round a shaft, a piston 40 for enlarging and reducing the capacity of a cylinder 39 by the reciprocating motion caused by the rotation of the cylinder block 31, a torque plate 34 which is inclined at a designated angle to the shaft center of the cylinder block 31 and with which the tip of the piston 40 is rotatably brought into spherical contact, and a rotation transmitting mechanism for rotating the cylinder block 31 and the torque plate 34 in synchronization with each other. A separate shoe for forming the spherical projected part of the tip of the piston 40 is provided, a smooth surface 45 brought into surface contact with the contact surface 44 intersecting perpendicularly to the shaft center of the piston main body 40a is formed on the shoe, and a cut-off surface 46 parallel to the smooth surface 45 is provided on the spherical surface tip of the shoe. A pressure vent groove 47 communicated with a case drain is formed on the spherical surface part of the shoe in parallel to the outer peripheral edge of the cut-off surface 46, and a communicating path 51 for guiding the operating pressure is provided on the cut-off surface 46.