PURPOSE: To straighten effectively a long-sized tube and bar stock by clamping and rotating one end of a long-sized stock to be straightened, which has been inserted and held by a position fixing roller and a straightening variable offset roller, and also running a clamp device.

CONSTITUTION: A straightening roller 2 and a clamp device 3 are arranged in one straight line with a fixing roller 1, a rolling tube stock from a cooling bed is inserted and held by each roller, and also the tube end is clamped by the clamp device 3. Subsequently, a steel tube is rotated by driving to rotate a skew roller, and also transferred by about half of a roller pitch in the longitudinal direction. During this time, the straightening roller 2 is offset and an offset is given to the steel tube and thereafter, the offset is returned and the steel tube is straightened. In this way, a long-sized steel tube can be straightened efficiently.