To increase the relative thermal capacity ratio of a piercing plug, to suppress the loss of the plug including the loss of the tip and surface roughening of the trunk part and to prolong the life by taking the crossing angle of inclined rolling rolls as the range of a specified angle and taking tube expanding ratio as not smaller than a specified value.

While rolling a billet 1 with the inclined rolling rolls 11, 21 which are oppositely equipped on both sides of the pass line of a solid billet 1 consisting of materials such as stainless steel, high alloy steel or carbon steel, a hollow piece 2 is obtained by inserting the piercing plug 14 into a round billet 1 in the longitudinal direction and a seamless tube is manufactured. The crossing angle γ of the inclined rolling rolls 11, 21 is taken as 0≤γ<5° and the expanding ratio as ≥1.15. Preferably, the piercing plug having a diameter larger than that of the round billet 1 is used. In the case the diameter of the round metallic rod piece is same and the piercing plug 14 having a larger diameter is used, even when the plug is brought into contact with the round metallic rod piece heated to a prescribed temp., the temp. of the piercing plug 14 is hard to be raised and softening is reduced.