PURPOSE: To prevent occurance of engine stop, by connecting the discharge conduit of a speed sensing pump to the pressurized chamber of a switch over valves, and by connecting the piston chamber of control valve units to tank T via switchover valves.

CONSTITUTION: The discharge conduit of a speed sensing pump P is connected to the pressurized chambers of switchover valves 9, and the piston chambers 6a of control valves 6 provided on the sides of push-pressing body of control valve units B are connected to tanks T by way of the switchover valves 9. Thus, when the driving torque of pump exceeds engine torque and the rotational speed of engine E drops less than a predetermined valve, the occurance of engine stop can be prevented in such a manner than the piston chambers 6a are released to the tanks T by switching the switchover valves 9, and the output of the pump elements A can be restrained by reducing the discharging amount in response to the discharging pressure.