PURPOSE: To obtain a thin steel tube of high out-of-roundness without correcting means by keeping that posture to a required temp. after rolling by arranging an elliptic caliber which is elliptic in the vertical direction in at least the final stand of rolling stands which are arranged in multistage.

CONSTITUTION: In the case the caliber (a) of the final stand is a normal circle, a steel tube (b) after rolling is deformed. As against that, because a caliber A is an elliptic caliber which is elliptic in the vertical direction, a period for generating deformation that a steel tube after deforming becomes a normal circle is about one minute in air cooling state. So the elliptic caliber with which the tube is held in the posture for this time just after rolling and becomes a normal circle as a result of deformation is arranged.