To prevent a machine frame from being raised when top soiling operation is performed by a hydraulic shovel.

A relief oil path 20 is branched from between a load check valve 5 installed before a cylinder 7 for raising and lowering a boom and a pressure compensating valve 4 installed on the secondary side of a variable choke 3, and a variable relief valve 21 is connected to the relief oil path 20. When a pilot valve 10 is operated to a lowering side in top soiling operation, a pressure switch 28 is turned on, and a solenoid switchover valve 24 is switched over to a 24b position by instruction signals from a controller 25 for a specified time period. Then a pilot pressure acting on the pilot port 21b of the variable relief valve 21 is vent to a tank 12, and the relief pressure of the variable relief valve 21 is changed to a lower relief pressure than usual. Even if a load pressure is increased by the contact of a bucket 14 to the ground, a delivery pressure of a hydraulic pump 1 reaches a pressure at which a machine frame is raised because the variable relief valve 21 is relieved.