A current load rate of an engine 10 is computed and a maximum absorption torque of at least one hydraulic pump 1, 2 is controlled so that the load rate is held at a target value. Engine stalling can be prevented by decreasing the maximum absorption torque of the hydraulic pump under a high-load condition. When an engine output lowers due to environmental changes, the use of poor fuel or other reasons, the maximum absorption torque of the hydraulic pump can be decreased without a lowering of the engine revolution speed. Further, the present invention is adaptable for any kinds of factors causing a lowering of the engine output, such as those factors that cannot be predicted in advance or are difficult to detect by sensors. In addition, because of no necessity of sensors, such as environment sensors, the manufacturing cost can be reduced.