Manufacturing method of seamless metal pipe, mandrel mill, and auxiliary tool

A manufacturing method of a seamless metal pipe which manufactures a seamless metal pipe from a hollow shell using none mandrel mill having a preceding-stage stand group including a plurality of stands arranged from a head along a pass line and a succeeding-stage stand group including a plurality of stands arranged behind the preceding-stage stand group, the manufacturing method comprising:

• preparing a plurality of mandrel bars for the one mandrel mill in which lengths of work portions which come into contact with the hollow shell during elongating are different from one another;
• selecting a mandrel bar including a work portion having a length corresponding to the number of stands used in thickness reduction, among the plurality of mandrel bars;
• inserting the selected mandrel bar into the hollow shell;
• and
• performing the elongating on the hollow shell into which the mandrel bar is inserted,
• wherein in the elongating, outer diameter reduction is performed by one of the preceding-stage stand group and the succeeding-stage stand group and the thickness reduction is performed by the other of the preceding-stage stand group and the succeeding-stage stand group, or the thickness reduction is performed by both of the preceding-stage stand group and the succeeding-stage stand group.