PURPOSE: To improve the life of a plug for piercing by specifying the relation between the diameter and effective length of the plug.

CONSTITUTION: The diameter of the plug for producing a seamless steel pipe, designated as Dp and the effective length of the plug, designated as Lp, are set at the relation of equation I. The radius r at the front end of the plug is set at ≥10mm and ≤30mm. The plug is formed of a casting alloy consisting, by weight, of 0.10 to 0.40% C, 0.10 to 1.00% Si, 0.20 to 2.00% Mn, ≤0.95% Cr, 0.50 to 5.00% Mo, 0.50 to 5.00% W, 0.10 to 5.00% Nb, and the balance Fe and unavoidable impurities and the surface of the plug is subjected to a treatment for sticking oxide scale. Further, ≥1 kinds among 0.50 to 5.00% Ni, 0.50 to 3.50% Co, 0.05 to 5.00% V, and 0.01 to 0.30% N are incorporated into this alloy. The life of the plug is further improved by adding the improvement of the plug shape and the material quality thereof.