In construction machinery, control is conducted to keep a work machine to operate at a constant speed regardless of the operation thereof. Different engine output lines T0-T3 and pump torque lines M1-M4 are defined for different operation types. All the matching points of the engine output lines T0-T3 and pump torque lines M1-M4 designate a constant engine speed N1. One engine output line and one pump torque line are selected according to the operation type presently selected. An output horsepower of the engine is controlled on an equal horsepower line corresponding to a sum of a horsepower for an auxiliary machine and a horsepower at a matching point of the selected lines. The pump torque for the work machine is controlled on the selected pump torque line. The engine operates at a constant speed N1 even when the operation type varies or the horsepower for the auxiliary machine changes.