PURPOSE: To automate the production of a curvilinearly bent pipe having a reduced diameter straight part, to improve production efficiency and to reduce a cost by pushing the seaming part of the reduced diameter straight part inward, thereby smoothing this part.

CONSTITUTION: The curvilinearly bent pipe is formed to a shape bisected by inner and outer peripheral walls and a work L integrally molded with a flange is set in the mating part. The reduced diameter straight part thereof is fitted onto a first core bar 400 to temporarily fix the work L. A second core bar 500 is then inserted into the work L from the other end of the work to separably connect the front ends of this core bar and the first core bar 400 until the core bars come into contact with the inside surface of the working point. The first and second core bars 400, 500 are then partly extended to be pressed to the inside surface of the working point and the flange is laid down alongside continuously from the one end side of the work, by which the flange is seamed. Further, the second core far 500 is retreated and the smoothing to push the seaming part of the reduced diameter straight part of the taken out work L inward is executed.