Provided is a tubular torsion beam for rear suspensions of vehicles, which is produced according to a more durable design capable of improving roll stiffness and roll strength of the tubular torsion beam. The tubular torsion beam is produced by pressure-forming a tubular steel member through hydroforming such that the tubular torsion beam has a cross-section varying along an entire length thereof, with opposite ends having a closed cross-section and mounted to respective trailing arms, a middle portion having a V-shaped open cross-section, and a transitional portion having a varying cross-section and connecting the middle portion to each of the opposite ends. During the process of manufacturing the tubular torsion beam, the opposite ends of the tubular steel member are fed using respective axial punches of a hydroforming machine, so that the opposite ends are thicker than the middle portion.