Abstract: Construction for uniting by welding a brake case cover, a rear wheel axle case and a terminal speed reduction unit case which are all made of sheet metal. The rear wheel axle case has at its opposite ends precisely circular tubular portions fitting in the brake case cover and in the terminal speed reduction unit case to connect them together. The united parts are joined together by automatic welding. The rear wheel axle case has a flat tubular intermediate portion for mounting a fender or the like thereon. The flat tubular portion has a smaller sectional area than the opposite end tubular portions so as to be substantially flexible and to thereby absorb concentric loads on the weld joints.