To provide a plug for improving the service life of the plug and the quality of the inside surface of a material to be rolled in the manufacture of a steel tube.

The plug which is used for manufacturing the steel tube is composed of a ceramic material or a cermet material containing the ceramic of ≥30% by volume ratio and the clearance between a through hole or a blind hole for connecting to a plug supporting rod inside the plug and the plug supporting rod is taken as 0.5-5 mm. Furthermore, the surface roughness of the through hole or the blind hole is taken as ≤25 μm. The plug is submitted to piercing work with a press piercer or rolling work with a plug mill. Consequently, the ceramic material and the cermet material which are excellent in seizure resistance and wear resistance are applied to the plug for manufacturing the steel tube practically without any problem and the service life of the plug and the quality of the inside surface of the material to be rolled are improved.