PURPOSE: To reduce the expansive tendency at the tube bottom part by detecting a diameter expansion amount at the tube bottom part generated by a mandrel remained in the bottom part after tube manufacture finishing and using a mandrel provided with a expanded head by a size corresponding to a diameter expansion amount.

CONSTITUTION: A diameter expansion amount of the bottom part generated by temporarily remained mandrel in the bottom part of a tube P is detected by comparing the amount with a diameter of the top and intermediate parts of the tube P. A mandrel 4, provided with a head 4b having a diameter corresponding to the expansion amount or a larger diameter than the corresponded diameter at the top end of the mandrel 4, is used to hot extrude the tube P. Therefore, a part facing the head 4b is cooled with the inside diameter of the facing part restrained from contraction and other parts contract to a position being in contact with the peripheral surface of a body part 4a without restraint. An extent of expansion is reduced by contraction after temporary expansion when the mandrel 4 is later pulled out because an endothermic amount is small.