Abstract: A selector or shift lever manipulates gears in a ground wheel transmission to select a running mode when moved in the transverse direction, and manipulates gears in a plow transmission to select the rotating direction of a rotary plow when moved in the vertical direction. A drag bar is mounted on a machine frame, slidable in the vertical direction, the vertical movement of the drag bar being interlocked with the vertical movement of the selector lever through rods and links. Hence, when the selector lever is moved downward to select forward rotation of the rotary plow, the drag bar is moved down into the ground in coordination, subjecting the soil tilling machine to great resistance from the ground, to enable a tilling operation to be performed while preventing lunging foward of the machine.