PURPOSE: To improve the durability of plug by providing the covering layer of a specified composition on the surface of plug for manufacturing hot seamless tube and further forming an iron oxide layer with a specified thickness on that surface.

CONSTITUTION: On the surface of the base material 2a of plug 2 which is brought into contact with a material to be worked for manufacturing hot seamless tube, the covering layer 2b in which, by wt.%, 0.1 - 1.5 % C, 0.1 - 1.0 % Si, 0.05 - 1.0 % Mn, 1.0 - 7.0 % Cr, 5.0 - 20.0 % Mo + W, 0.5 - 5.0 % V and the balance Fe and inevitable impurities are contained is formed and the iron oxide layer with the thickness of 30 - 300 μm is formed on that surface. In this way, since the high-temp. strength of plug can be improved and the thermal insulation property and lubricity can be provided, the durability can be improved.