PURPOSE: To realize energy recovery by providing a discharge adjusting means to a discharge chamber which is a discharge means for adjustably supplying a high-pressure fluid to a region selected from unmeshed regions of mutually meshing teeth.

CONSTITUTION: A spool 30 is disposed across mutually meshing regions between a suction chamber 26 and a discharge chamber 28. Slots 32, 34 formed in the spool 30 are respectively on a discharge side and a suction side. Even when the spool 30 is in a right limit position, namely the total amount discharge position, the slot 32 communicates the meshing region of gears 22 and 24 in order to make the pressure of the meshing region equal to the pressure in the discharge chamber 28. When the spool 30 moves laterally, the slot 32 lies across the entire meshing region of gear teeth and partially extends into an unmeshed region till the left side of a center line M, thus being in the state of recovering energy.