PURPOSE: To improve a peeling limit, and to reduce the generation factor of a guide shoe mark by that the projection of a piercing shell between a main roll and a guide shoe, becomes less.

CONSTITUTION: When piercing a seamless steel pipe by the Mannesman system, a relative position in the perpendicular direction of a main roll 1 and a piercing shell 3 in a gorge position B, is allowed to offset on the side where the piercing shell 3 projects between the main roll 1 and a guide shoe 2 by the rotations of respective main rolls 1. And, piercing is performed at a state in which a gap between the main roll 1 and the guide shoe 2 on the side where the piercing shell 3 projects, is small.