In a thin-wall type ball nut of a light-weight type ball screw having a trunnion structure in which a pair of trunnion shafts are extended from the ball nut in the opposite directions; a pair of ribs for increasing the wall thickness of the ball nut are formed on both side surfaces of the ball nut in such a manner that the ribs are extended from the base portions of the trunnion shafts along the axis of the ball nut, so as to increase the rigidity of the portions of the nut body which are liable to be deformed, thereby to prevent the ball nut from deformation.
