PURPOSE: To prevent the generation of interior defect due to tube making by making the tip side of a mandrel bar for manufacturing seamless steel tube smaller than the reference diameter and connecting with the reference diameter part through the taper part.

CONSTITUTION: At the time of manufacturing a seamless steel tube by Mannesmann mandrel mill process, a smaller part 11 of which the diameter D1 is smaller than the reference diameter D is provided on the side of the tip and the mandrel 10 is composed by connecting with the reference diameter part 12 at the middle or the reference side with this smaller diameter part 11 through the taper part 13. In this way, at the time of tube making by Mannesmann mandrel mill process, the interior defect of tube is prevented and the life of the mandrel is improved.