PURPOSE: To enable uniform tilling of the surface of a field independent of the soil condition by providing a controlling means for varying the width of the neutral zone of depth control according to the extent of the lateral inclination of the ground- working machine.
CONSTITUTION: The width D of the neutral zone of depth control is determined by a controller 27 based on the signal transmitted from an inclination detector 16. The detected value transmitted from a tilling depth detector 37 of a depth controlling means is compared with a preset depth value set by a tilling depth setting device 40. A lifting command is outputted to a proportional control valve for lifting 46 when the difference X is larger than the width D of the neutral zone; the control valve is kept in neutral state when the difference X is within the range of -D<X<D; and a lowering command is outputted to a proportional control valve for lowering 47 when X≤-D. Furthermore, the length (l) of a hydraulic cylinder 15a necessary to keep a rotary tilling apparatus 14 in horizontal state is calculated, the solenoid of the extension-side control valve 48 is excited when the length (l) is smaller than the detected value STS of a stroke detector 7, the system is maintained in neutral state when the length is equal to STS and the solenoid of the contraction-side control valve 49 is excited when (l)>STS.