Abstract: A roll control structure adapted to automatically control an earth working machine, such as a rotary cultivator so that the machine becomes horizontal in the lateral direction irrespective of the inclination of the ground surface, i.e., in such a manner that the machine assumes a posture preset by the operator. The roll control structure is characterized in that it is provided with left and right stroke sensors for detecting strokes, i.e. a distance between arbitrary points on the left and right lift arms in an earth working implement connecting link mechanism and those on the left and right lower links in the same mechanism to determine an angle of inclination in the lateral direction of the implement with respect to a tractor on the basis of a difference between the detected strokes.