PROBLEM TO BE SOLVED: To provide a device for optimizing an automatic tilling-depth control in accordance with the condition of a farm field.SOLUTION: A tractor control device 19 controls the lift of a tiller 3 on the basis of the detection result of a depth sensor 8 so that the tiller 3 connected to a running machine body 1 may be a set tilling depth. The control device 19 causes a target value or a reference to the lifting output to follow the detected value of the depth sensor 8 at a low speed, and calculates the upper or lower limit of an insensitive band on the rising side or the falling side by adding or subtracting a predetermined value to or from that target value. If the detected value of the depth sensor 8 deviates from the upper or lower limit of the insensitive band, a rising or falling output of the tiller 3 is performed.
