PURPOSE: To prevent the thickness increase phenomenon at the forward and rear ends of a pipe by beforehand reducing the outside diameter at the pipe end in front of a reheating furnace, welding both base pipes between the reheating furnace and a stretch reducer, and cooling the coupled part to a specific temp. just prior to rolling.

CONSTITUTION: The outside diameter of a pipe end is reduced to the diameter smaller than the caliber diameter of the final stand of a stretch reducer in the stage of reducing the metallic base pipe with the stretch reducer. The pipe end is reduced to about 1/3 the diameter of the pipe at the largest. Thereafter, the reduced end parts of the base pipe are butted and joined by welding between a reheating furnace and the stretch reducer. The coupled part is cooled to the temp. lower by ≥50°C than the temp. of the base pipes just prior to the rolling. The thickness increase phenomenon at the forward and rear ends of the product is thus prevented in the stage of producing a circular pipe of small diameter.