PURPOSE: To prevent occurrence of such a condition that hydraulic oil is fed into a hydraulic actuator by providing control means for feeding hydraulic oil into a negative brake so as to turn the negative brake onto the control releasing side.

CONSTITUTION: In such a condition that a negative brake falls in a braking condition, a left control lever 9 is turned toward a right turn position R by a little, or on the contrary, it is turned toward a left turn position L by a little, this operation is detected by a potentiometer 19, and accordingly, a control device 18 changes over a selector valve 17 into a supply position. Thus, a high pilot pressure is fed from a pilot pump 13 into the negative brake 15 through the selector valve 17 and a check valve 20 so as to turn the negative brake 15 onto the brake releasing side with no delay from the turning of the left control lever 9 toward the right or left turn position R or L.