PURPOSE: To shorten a span of hydraulic force down time of a discharge line by taking pressure oil in the pump discharge line into a pressure chamber, controlling a variable element and simultaneously making the pressure oil in the discharge so as to be interconnected to a tank directly.

CONSTITUTION: With movement of a proportional solenoid four-way control valve 5 to the right side in illustration, it makes a pilot line LP get opening gradually to a pump discharge line PP, and if a control input current runs short, the pilot line LP is fully opened by the pump discharge line PP. Then, a pump discharge pressure is conducted into a pressure chamber 4a via the proportional solenoid four-way control valve 5 and a safety valve 6, while an operating piston 4 is shifted in opposition to force of a spring 3, and a variable element 2 is displaced so as to be turned to a full-cutoff position from a maximum discharge position. Simultaneously with this, pressure oil from the pump discharge line PP is dropped into a tank via a differential pressure valve 10 and the proportional solenoid four-way control valve 5, thus a pressure drop in the pump discharge line PP is thus accelerable. With this constitution, hydraulic force in the pump discharge line PP can be lowered in a short time.