PURPOSE: To avoid the increase in the working degree in the two roll stand rows, and to reduce the bar changing work and the cost by changing the wall thickness as large as the wall thickness change of a finished tube on the stock tube side to be fed in two roll stand rows.

CONSTITUTION: A pair of grooved rolls 11 whose screw-down direction is orthogonal to each other and four roll stands 10 whose screw-down direction Is deviated by 45° are arranged, a stock tube 20 where a core rod 30 is inserted is fed to the stand rows, and the wall thickness is reduced by the two roll stand rows, and the wall thickness fluctuation in the circumferential direction is reduced by the four roll stands 10 respectively. When the finished tube of the different wall thickness is rolled by tightening the roll gap by the two roll stand rows using the core rod 30 of the same outer diameter, the wall thickness change as large as that of the wall thickness change of the finished tube is achieved on the side of the stock tube to be fed to suppress the increase in the working degree by the two roll stand rows. The peak thickness nonuniformity at four positions of 45° from the groove bottom which is generated in the two rolling operations is eliminated thereby, to reduce the bar changing work and the cost.