PROBLEM TO BE SOLVED: To provide a fixing structure of a tubular member, which prevents a mouthpiece body from rotating due to torque generated when attaching or detaching an external cap.SOLUTION: A fixing structure of a mouthpiece body 31 comprises: a through hole 22h, which is provided in a case body 22, for communicating an internal space 12S and the outside a small diameter part 31b which has an externally exposed part 31c, in which a first male screw 31cm is formed, and an exposed part 31d exposed to the inner space side, which is provided continuously to the externally exposed part 31c, and in which a rotation restriction face 31f is provided in the externally exposed part 31c a first nut 41 which is screwed to the first male screw 31cm and can advance and retract freely a positioning hole 23h having a bottom face 23d and a positioning face 23f a rotation stopper member 50 having an outside fitting face 52, which is disposed in the positioning hole 23h and is in contact with the positioning face 23f, and an inside fitting face 51 in contact with the rotation restriction face 31f of the externally exposed part 31c and a second nut 42 which is mounted to the exposed part 31d exposed to the inner space side and restricts the small diameter part 31b from coming out to the outside of the through hole 22h.SELECTED DRAWING: Figure 8C
