PURPOSE: To prevent a fluid from leaking to the outside as well as make improvements in operating efficiency by supporting a port plate on a cylinder block, and installing a hollow balance piston in a port.

CONSTITUTION: Each port plate 20 is attached to cylinder blocks 3, 6 displaceably in the axial direction. Each port 23 being interconnected to each cylinder 4 is formed as piercing through this port plate 20. Each hollow balance piston 25 being elastically supported by springs 26 is installed in each of these ports 23. Expansion or contraction of the balance piston 25 compensates an axial relative displacement between each part of these cylinder blocks 3, 6 and a port block 9. Thus, a fluid can be prevented from leaking to the outside between the cylinder block and the port block.