PURPOSE: To lower the percentage of thickness deviation by applying outside diameter working intended primarily for correcting thickness deviation with a 2-roll type skew rolling mill having no inside surface regulating tools prior to a process of mainly working wall thickness with an elongator.

CONSTITUTION: A billet 10 is heated in a heating furnace 1 of rotary hearth type, and is pierced with a piercing mill 2 to a hollow blank pipe 11 which is next passed through a skew rolling mill 3 of 2 roll type having no inside surface regulating tools such as plugs or mandrel bars. This rolling mill 3 works the outside diameter of the pipe 11 mainly for the purpose of correcting its thickness deviation, and is inclinedly disposed with 2 pieces of rolls of a truncated circular cone shape or a barrel shape. The pipe 11' reduced of diameter and corrected of thickness deviation by this mill 3 is passed through a mandrel mill and is thereby made into a semiprocessed pipe which is thence heated in a reheating furnace and is passed through rolling mills, whereby it is made into a product.