PURPOSE: To use effectively the output of a prime mover by controlling the discharg of a hydraulic pump in response to discharge signal generated by a signal output means for generating discharge signal in response to rack position deviation and based upon the maximum operating amount of an operating lever.

CONSTITUTION: When a load is increased to reduce the rotational frequency of a prime mover 1, change, i.e. deviation in the rack position is detected by a deviation detector 27 to send rack position deviation signal output ΔR to a function generator 28. When ΔR is small, it is 1. However, when it exceeds a predetermined value, discharge signal output Y2 having reduced value corresponding to ΔR is sent to a signal comparator 30 which compares discharge signal Y1 with discharge signal Y2 to send smaller value of them to a regulator 6 as discharge signal output for controlling the discharge of a pump 2. Thus, the discharge has a value corresponding to the discharge signal Y1 based on the maximum operating amount of operating levers 10W12. Thus, said discharge has the necessary minimum value to minimize the output of the prime mover.