Abstract: A power assistance steering system for use in a vehicle comprises a restricting mechanism including a cylindrical reaction piston having a reaction surface on which said reaction pressure is applied in order to cause said reaction piston to be biased downwardly, and at least one reversed V-shaped depression formed on a bottom skirt, the cylindrical reaction piston adapted to be held on the input shaft in such a way that said reaction piston can longitudinally and slidably reciprocate with respect to the input shaft, while circumferentially locked to the input shaft, and at least one semi-spherical protrusion formed on a middle end surface of said output shaft so as to be retained within the reversed V-shaped depression.