While a material pipe and a roller are revolved relative to each other with a predetermined radius, the material pipe and the roller are moved relative to each other in the direction of progress of forming by eccentrically moving the center axis of the material pipe and the revolution axis of the roller relative to each other and simultaneously moving the material pipe and the roller relative to each other in the directions of the axis. During movement in the direction of progress of the forming, an end portion of the material pipe is deflected by controlling the axis-direction moving portion so that the roller is moved relative to the material pipe in the direction of the axis when the circumferential position of the roller, with respect to a circumference of the material pipe, changes from an inner side of the direction of deflection toward an outer side of the direction of deflection. This allows the movement of the roller relative to the material pipe in the direction of the axis to be stopped when the circumferential position of the roller changes from the outer side of the direction of deflection toward the inner side of the direction of deflection.