PURPOSE: To make a pump discharge quantity accurately agree wit a target discharge quantity by providing a flow rate detecting motor on the discharge line of a pump and feeding back the rotating speed of said motor to a displacement quantity adjusting means.

CONSTITUTION: A flow rate detecting motor 3 is provided on the discharge line 13 of a pump 1, and the rotating speed of the motor 3 is inputted into a controller 5 via a flow rate feedback means 4. Thereby, an actual discharge quantity can be accurately detected by the flow rate detecting motor 3, while enabling the pump discharge quantity to agree accurately with a target discharge quantity regardless of the leakage of oil or change in viscosity in the actual discharge quantity, due to the feedback of the actual discharge quantity.