A fluid delivery adapter unit for selectively connecting fluid supply equipments to an endoscope comprises a straight pipe capable of being detachably coupled to a fluid inlet collet of a headpiece of the endoscope and a generally L-shaped elbow pipe having a lure lock component at a proximal end to which the fluid supply equipment is coupled. A joining structure is installed between the elbow pipe and the straight pipe so as to allow the elbow pipe and the straight pipe to turn through a predetermined angle relative to each other only when the elbow pipe is forced to turn and further to keep them in a turned position while the elbow pipe is unforced. The joining structure comprises principally involves an elastic sealing member to provide fluid tight seal between the elbow pipe and the straight pipe.