PROBLEM TO BE SOLVED: To provide a treatment instrument insertion section of an endoscope which enables to connect a proximal nozzle of the treatment instrument insertion channel to a treatment instrument insertion nozzle within an attaching hole of the treatment instrument insertion nozzle formed in an operation section housing and to fix the nozzles to the operation section housing easily and credibly without generating looseness.SOLUTION: A connecting member 10 arranged on the distal part of the proximal nozzle 4 of the treatment insertion channel includes a plurality fence-like pieces 11 made of an elastic material and projecting from a plural positions of the outer circumference in the direction parallel to an axis. Each fence-like piece 11 has a loosing prevention pawl 12 projecting inward from the distal end of the fence-like piece. Hole-side engaging grooves 8 extending parallel to the axis are formed on the inner circumference of the attaching hole 2 of the treatment instrument insertion nozzle, and connecting part engaging grooves 9 extending parallel to the axis are formed on the outer circumference of the treatment instrument insertion nozzle 7. A male threaded part 7s that threadedly engages with a fixing nut 13 is formed on the outer circumference of the treatment instrument insertion nozzle 7.
