The utility model provides a can appropriately prevent the mark and pipe for radiography towards the offset of axis direction. Radiography ring (30) is installed in the periphery side of the interior side pipe portion (16) of sacculus pipe. Radiography ring (30) possesses main part (31) and protruding portion (32). Main part (31) are cylindric parts. Protruding portion (32) are the outstanding parts of inner peripheral surface from main part (31). Protruding portion (32) form in the whole region of circumference of main part (31) with the orientation of the axis direction quadrature with main part (31) in succession. Under the state that the main part (31) and the outer peripheral face of interior side pipe portion (16) are hugged closely, install radiography ring (30) in interior side pipe portion (16). Protruding portion (32) under the state of side pipe portion (16), encircles (30) with the radiography and installs in interior side pipe portion (16) in being nipped from periphery side.
