PURPOSE: To reduce the cost of oil-pressure shovel as well as prevent the occurrence of negative pressure and breathing phenomenon by connecting the bottom side chamber of the first oil-pressure cylinder with a tank through a check valve and the neutral position of the switch valve of the second oil-pressure cylinder.

CONSTITUTION: Bottom side chamber to cause shortage of oil amount in a bucket cylinder 9 or boom cylinder 7 is connected to a tank 15 through a combination valve 14E, check valves 25 and 26, and pipelines 23 and 24. When the combination valve 14E is in neutral, the pipelines 23 and 24 are connected to the tank 15. At the left position 14E1 of the valve 14E, pressure oil is supplied to the boom cylinder 7 by the oil-pressure pump 11 for acceleration, and also at the right position 14E2, pressure oil is supplied to the arm cylinder 9 by the pump 11 for acceleration. The occurrence of breathing phenomenon can thus be prevented and also the cost can be reduced.