Engine lag down suppressing device of construction machinery

To control small an engine lag down subsequent to a lapse of a predetermined holding time during which a low pump torque is supposed to be held upon operation of a control device from a non-operated state, an engine lag down control system for construction machinery is provided with a machinery body controller 13 having a first torque control means and a second torque control means, a solenoid valve 16 and the like, and a third torque control means. The first torque control means controls a torque control valve 7 to a minimum pump torque (value: Min) corresponding to a target number of engine revolutions Nr when a non-operated state of a control device 5 has continued beyond a monitoring time TX1. The second torque control means controls the torque control valve 7 such that the above-described minimum pump torque is held for a predetermined holding time TX2 subsequent to the operation of the control device 5 from the non-operated state. The third torque control means controls the torque control valve 7 such that from a time point of a lapse of the predetermined holding time TX2, the pump torque is gradually increased on a basis of a predetermined torque increment rate K as time goes on.