To provide a fixed plug for manufacturing a seamless steel tube, which has such a simple structure that attachment and detachment of a plug used for manufacturing a hot seamless steel tube is carried out easily in a short period of time when fixing the plug to a mandrel bar, and which reduces a burden of working labor.

The fixed plug for manufacturing a seamless steel tube is a fixed plug for a piercer. A groove part is formed on the inner circumferential surface of the fixed plug for a piercer, and a key is formed on the front end part of a mandrel bar cap. Thereby, the plug and the mandrel bar are integrally fitted and fixed to each other.