PURPOSE: To conform the elevation of a boom and its slewing speed regardless of loads of the boom by controlling the maximum discharge pressure of a variable oil-pressure pump for driving a slewing motor by correction with a set value corresponding to the discharge pressure of the variable oil-pressure pump and the driving pressure of the boom.

CONSTITUTION: A solenoid valve 9 is provided between a boom cylinder 8 and an oil-pressure pump 7 for driving the cylinder 8, and a solenoid valve 4 is provided between a turning oil-pressure motor 3 and a variable oil-pressure pump 1 for driving the motor 3. The discharge pressure of the pump 1 is detected by a detector 5 and put in a controller 12, the discharge pressure of the pump 7 is detected by a detector 10 and put in the controller 12, and the maximum discharge pressure of the pump 1 to drive the motor 3 is corrected by the drive pressure of the boom and also controlled by a set value corresponding to discharge pressures. Conformation of the elevation of the boom and its slewing speed can thus be attained regardless of forces to be imparted to the boom.