PURPOSE: To enable to effect the constant horse power control simply by a method wherein the load current of an induction motor driving the hydraulic pump is detected to control the discharging quantity of the same pump.

CONSTITUTION: The load current of the three-phase induction motor 2 for the hydraulic pump 3 is detected by a current transformer 6. The output thereof is converted into a DC voltage by an AC/DC convertor 7. The DC voltage and the voltage of a horse power settor 8 are compared and substracted in an operational amplifier 9 and the output thereof is inputted into an integrator 10. The output of said integrator 10 is inputted into a servo amplifier 11 so as to decrease the discharging quantity while the output thereof controls the hydraulic pump 3 by a hydraulic pump control system 4 and decreases the discharging quantity. When the hydraulic pump 3 decreases the discharging quantity thereof in accordance with the input, the load current of the three-phase induction motor 2 is decreased and the output voltage of the integrator 10 is balanced at a position which is balancing with the load current.