PURPOSE: To increase the thickness of a pipe body end part without generating a wrinkle-like flaw on the inside face, by constituting so that a swaging end part of a mandrel bar which is made to press-contact with the end face of a blank pipe is shaped like a taper inclined to the outside and the inside in the counter pressure direction.

CONSTITUTION: An end face 13 of a swaging end part 12 of a mandrel bar 11 is formed by a tapered outside circumferential side end face 13A which can be made to press- contact with the outside circumferential side end face of a hollow blank pipe 1 and is inclined in the counter pressure direction, and a tapered inside circumferential side end face 13B which can be made to press-contact with the inside circumferential side end face of the hollow blank pipe 1 and is inclined in the counter pressure direction. That is to say, it is formed like a wedge in which the tapered outside circumferential end face 13A and the tapered inside circumferential end face 13B cross each other at a prescribed angle. Accordingly, resultant force F of frictional force F1 which operates between the end face of the hollow blank pipe 1 and the tapered inside circumferential side end face 13B, and vertical pressure F2 is nearly directed in the pipe axis direction, compressive stress of one axis operates on the hollow blank pipe 1, and depth of a wrinkle flaw is reduced to shallow.