PURPOSE: To simplify circuit construction and enhance workability at the time of assembling by controlling the discharge of pressure oil, which is discharged from a hydraulic pump of a variable delivery type, for being variable for a plurality of stages.

CONSTITUTION: The pressure, which is generated in a center by-pass pipe passage 7, is taken out in a pilot pipe passage 9 as the pilot pressure. At the same time, the pilot pressure in a pilot pipe passage 31 is controlled by an electromagnetic, proportional reducing valve 32, based on signals from selection switches 33, 34. In addition, the pilot pressure in either pilot pipe passage 9 or 31 whichever is on a higher side is supplied to a servo cylinder 2 in order to control the discharge from a hydraulic pump 1.