PURPOSE: To improve a controlling accuracy of rolling, especially a dimensional accuracy of the 1st pipe of a lot, by performing the calculation of screw-down setting using with actually measured values resulted from actual measuring of the fluctuation of the outer diameter of a plug, during rolling.

CONSTITUTION: A pair of plug-rotating rollers 12 are provided to a plug measuring device 22 to measure the outer diameters of a plug from the plural measuring positions by rotating the plug 3 around its axis in a V-shaped slot 21 by means of the rollers 12. Further at the same time, the outer surface of the plug 3 is cleaned by a scale breaker 13 equipped with a brush, etc. to measure the outer dia. accurately. The plug 3 contained in the slot 21 drops by its own- weight into a V-shaped trough 14 located underside of a pass line by a rotation of a drum 32, and is fitted to a plug bar 8 to be inserted into between upper and lower rolls 27. Expanding and elongating of a blank pipe by rolling are performed by using said plug and rolling rolls.