To reduce an operating force by reducing the strength of a return force exerting on an operation part when the operation amount of the operation part exceeds a predetermined value.

The engine control system of a tractor 1 has its throttle lever 12 and engine throttle 11 operated with the throttle lever 12 so that they are connected through a control rod 10; when the control rod 10 moves in one direction through the operation of the throttle lever 12, the return force of a return spring 24 coupled with the control rod 20 operates associatively. Then the return force of the return spring 24 increases as the movement quantity of the control rod 10 increases, but when the movement quantity exceeds a previously set value, return force adjusting mechanism 18 and 26, etc., reduce the strength of the return force of the return spring 24.