PURPOSE: To prevent the generation of roll mark and obtain butted tubes without surface flaw by changing the ellipticity of a raw tube at the former stage of the last stand to reduce the outer diameter, tubing the outer diameter of the pretreated tube like the ellipse with the required ellipticity at the last stand and passing it through the strainer.

CONSTITUTION: When a heated skelp is butt-welded by the butt-welding machine to make a tube, the outer diameter is reduced while changing the ellipticity of the tube at the each stand located on the former stage of the last stand and the outer diameter of the pretreated tube is made an ellipse with the required ellipticity at the last stand and then passed to the strainer through the sizer or not to make a complete butted tube while applying clashes forcedly. Thus, the generation of roll marks can be prevented during the process of reducing butted tubes and the surface roughness on the position corresponding to roll marks is reduced up to 1/2W1/5 in comparison with the conventional products.