PURPOSE: To equalize a surface and thickness of a corrugated part, by rotating a material pipe, and making a freely rotatable forming roller press-contact to said pipe.

CONSTITUTION: A material pipe 1 is rotated, and a forming roller 2 which is freely rotatable and has been placed at a right angle position with an axis B of the material pipe 1 is made to press-contact to said material pipe 1. Subsequently, on the material pipe 1, a corrugated part 3 is formed. In this way, it is possible to obtain a pipe whose thickness is not decreased even in its corrugated part, is uniform, and whose formed surface is also uniform.