A first ring having greater hardness than piping and hardness that is equal to or less than that of a joint, and a second ring having greater hardness than the joint are inserted through the piping. The piping is inserted into a fastening member, and the tip end of the piping is inserted through a first recessed section of the joint. By screwing a threaded section of the fastening member into a threaded section of the joint, the first ring and the second ring, which are sandwiched between a first contact surface provided to the first recessed section of the joint and a second contact surface provided to a second recessed section of the fastening member, are pressed toward the first contact surface and fix the piping by digging into the piping. Furthermore, the piping, which has been fixedby the first ring and the second ring, moves to the joint side by a pressing force, and presses an orifice plate installed on an installation surface of the joint toward the installation surface sideby the tip end of the piping. By doing so, it is possible to maintain the sealing properties of a flow path inside the joint at a plurality of locations including the orifice plate in cases where thepiping is connected to the joint on which the orifice is to be arranged.