A method enhances fatigue strength of ear portions which are high fatigue risk parts in a torsion beam almost without bringing about disadvantages in terms of productivity and cost. A torsion beam is formed with a portion of the tubular body into an approximately U-shape in cross section by crushing the portion of the tubular body in the radial direction and, thereafter, due to bending with a bottom line set as a belly side of bending, a bending strain of 2 to 6% in the tube longitudinal direction on a tensile side is imparted to the ear portions.