A screw shaft device includes a screw shaft having an outer helical groove and having an intermediate portion, the screw shaft includes a longitudinal bore, a ball nut is threaded with helical groove of the screw shaft, and a number of ball bearings or rolling members are disposed between the ball nut and the screw shaft for facilitating a movement of the ball nut relative to the screw shaft, an inserted member is disposed in the intermediate portion of the longitudinal bore of the screw shaft, and the other inserted members are disposed in the end portions of the screw shaft, and the inserted member are made of different materials.