To eliminate constant outflow of pressure oil from a bleed-off valve due to a momentary pressure change so as to minimize an energy loss in the whole of a hydraulic driving system.

A hydraulic driving system provided with a bleed-off valve 24 is provided with a valve unit 30. The valve unit 30, which is connected to a discharge pipeline 3 and a pilot line 12, responds to a differential pressure fluctuation due to a momentary pressure fluctuation in the discharge pipeline 3 and the pilot line 12 and discharges a part of a discharge quantity of a hydraulic pump 1 before the bleed-off valve 24 is opened when the pressure exceeds the third set pressure or the fourth set pressure lower than the third set pressure.