PURPOSE: To enable using a mandrel to the use limit due to wear of a nitrided layer by having the nitrided layer of which the depth from the surface on the outer layer part except its thread part of the mandrel is deeper than a specified value and the surface hardness is a specified value.

CONSTITUTION: The mandrel for hot extrusion has a thread part B for mounting itself to a mandrel holder on the base end part and consists of a hot alloy tool steel which is adjusted to a hardness of HRC 40 to 55 by quenching and annealing. The outer layer except the thread part of this mandrel A has the nitrided layer of which the depth from the surface is ≥0.25mm and the surface hardness of the nitrided layer is defined as HV 1000 to 1100. Using the hot alloy tool steel, the mandrel is machined, a cap is mounted on the thread part of the mandrel and the mandrel is heating treated in the atmosphere of gaseous ammonia. In this way, the peeling of the nitrided layer and the breaking of the mandrel in the case that the nitrided layer is formed on the outer layer of the mandrel are prevented, so a long life is attained even when the mandrel is used for extruding a hard-to-work material such as a high alloy.