there is provided a processing method of a tube body comprising the steps of : a tube - expanding step wherein a base tube is partially retained with a chuck unit having a rotational function , and a mandrel is pressed in to an end portion of the base tube so as to expand the end portion of the base tube ; and a rotation ironing step wherein the chuck unit rotates the base tube together with the mandrel , a roller die is pressed against an outer - periphery surface of the end portion of the base tube and the roller die is moved with respect to the base tube and in an axial direction thereof so as to deform an inner - periphery surface of the end portion of the base tube into a shape corresponding to an external shape of the mandrel .