PURPOSE: To make the wall thickness of a final product uniform in its longitudinal direction, by controlling the position of a rolling plug while a rolling is operated in an elongator so that a rolling load is kept constant at the time of performing one pass of rolling in a succeeding plug mill.

CONSTITUTION: An elongator has skew rolls 11 facing to each other, and rolls further a hollow pipe 14 formed into hollow one between a rolling plug 13 attached to the front end of a plug bar 12 and the skew rolls 11 in a piercer, and reduces the wall thickness of the pipe 14 by expanding and elongating it. At that time, the position of the plug 13 of a skew rolling mill is adjusted at the time of rolling a succeeding material so that the change with time of the rolling load of a plug mill for the succeeding material is zero, by estimating the change with time of the rolling load of the plug mill imposed on the pipe ranging from its front end to its rear end, obtained from the preceding material, is equal to the change with time of the rolling load of the plug mill for the succeeding material when assumed that the succeeding material is pierced by the same condition as that of the preceding one.