To enhance safety by letting a tube falling prevention member function for preventing a chuck member from being released after a tube has been inserted and pinch-locked by the chuck member, and thereby preventing the tube from being carelessly fallen out because of tube inner pressure during dental treatment.

A tube 20 is connected to a quick disconnect tube coupling 10. Subsequently, a chuck releasing member 12 is returned to the opposite direction of an arrow head A, and a spacer member 30 is interposed between the tube end of the tube coupling 10 and the collar part 12a of the chuck releasing member 12. The interposition of the spacer member 30 prohibits the chuck releasing member 30 from being moved even if pressing force is applied to the chuck releasing member 12 in the direction indicated by an arrow head B. Therefore, the chuck will not be opened. The spacer member 30 is so constructed that an elastic member L width roughly equivalent to a clearance W appearing between the collar part 12a of the chuck releasing member 12 and the tube end of the tube coupling 10 when the chuck releasing member 12 is retreated, is formed out of end parts 32 which are elastically opened when the fitting is mounted/demounted, and of a grip part 31.