PURPOSE: To facilitate volumetric control of liquid through control of reversing valves with minimum power loss.

CONSTITUTION: Rotors of volumetric type hydraulic machines 1, 2 which are available as pump or motor are interlocked with each other through a main shaft 11 and reversing valves 22, 23 are connected with each other through conduits (18, 19), (20, 21) between inlets 7, 8 and outlets 9, 10 of both machines. A conduit 24 connected with a fluid source is divided into two conduits 25, 26 on its way, and their tips are connected with the reversing valves 22, 23 and the tips of conduits 27, 28 branching off a main exhaust conduit 29 are also connected with the valves 22, 23. When the reversing valve 22 is placed at a point a and the reversingvalve 23 at a point d, the machine 2 alone functions as a pump with a discharge ratio of 1/4.