PURPOSE: To almost maintain the outer diameter of a part passing through a die and that of a part after passing through the die at specified values to equalize the diameter of the middle part by making the outer diameter of a mandrel smaller toward the rear end side since the thermal expansion of the mandrel at the part passing through the die becomes remarkable with the advance of extrusion.

CONSTITUTION: A heated hollow billet 3 is inserted into a container 1 and this billet is extruded to the front from a gap between the die 2 positioned before this container 1 and the mandrel 4 inserted by penetrating the billet 3 into this die 2 to make a tube. A tapered mandrel in which a part on the rear end side of a part in contact with the billet 3 upset at least at an upset time is reduced in diameter gradually toward the rear end side is used as this mandrel 4 so that a tube extruded in front of the mandrel is equalized in diameter in the axial direction at a completion time of extrusion. This method can restrain a diameter expanding phenomenon in the middle part of an extruded tube contributes to improvement of the dimensional accuracy of the extruded tube and prevents reduction of yield by seizure.