PURPOSE: To reduce the manufacturing and running costs of a mandrel bar by dividing the mandrel bar into working bars directly necessary to rolling and a dummy bar different from them and sharing the dummy bar.

CONSTITUTION: When the dummy bar 8 is rolled by the mandrel mill 5, its outside diameter is unnecessitated to be equal to that of the working bars 7. However, since the dummy bar enters the mandrel mill 5, it must take a circular section. Consequently, one dummy bar 8 can join to plural working bars 7 having a little different diameters. Some working bars 7 are damaged because they are rolled together with a shell 4 and when the damage grows, they are disused. Therefore, the working bars are manufactured in consideration of the number of the disused working bars. As a result, since one dummy bar can be manufactured to plural working bars, the manufacturing and running costs are reduced compared with that of a long mandrel bar.