PURPOSE: To maintain a rotational speed at a fixed value at idling time of an engine.

CONSTITUTION: In a rack position detecting part 20, an actual rack position Rr is detected at idling time of a control rack 10a. In a differential arithmetic part 27, a correction value ΔRc is calculated and output to a correcting part 14 based on a difference between a predetermined reference rack position and a mean value RrL of the actual rack position Rr. In the correcting part 14, a target drive rack position Rd' is calculated by subtracting the correction value ΔRc from a target drive rack position Rd at idling time. In a PID arithmetic part 17, PID calculation is performed based on a deviation between the actual speed N and target idle speed Ni of an engine. This arithmetic result Rp and the target drive rack position Rd' are added, to set this addition value to a target rack position RD at idling time.