PURPOSE: To improve the quality of a round billet to be rolled by providing a tension detector to a holding device of a mandrel bar and making the gap between barrl-type rolls changeable in accordnace with the vairation of the tension of mandrel bar.
CONSTITUTION: A tension detector 10 detects a tension TB applied upon a mandrel bar 1 to send a detecting signal S1 to a calculator 12. The calculator 12 operates further a detecting signal S2 of a load cell 11, a detecting signal S3 of a motor current detector 13, a detecting signal S4 of a motor number of revolution detector 14, and a detecting signal S5 of a motor voltage detector 15 to send its operated signals to a motor electric source 28 through a rolling reduction controlling device 27 of a rolling reduction device 23b, thereby the voltage of a motor 22 is controlled so as to attain a tension value set by a tension setting device 16. In this way, the rotational of motor 22 is controlled thereby the roll gap between barrel-type rolls 3a, 3b is controlled by the devices 23a, 23b.