PROBLEM TO BE SOLVED: To suppress the protrusion quantity of an observation window from the leading end of an insertion part and to enhance the washing properties and drainage properties of the observation window.SOLUTION: An air sending/water sending nozzle 22 and the observation window 28 are provided to the leading end part 16a provided to the leading end of the insertion part of an endoscope. The observation window 28 has the convex lens surface 34 protruded from a flat surface 27c and a jet orifice 38 is positioned on the first straight line Lconnecting the apex T of the convex lens surface 34 and the downstream end Ein the jet direction of a fluid in the convex lens surface 34 and the jet direction S when the fluid is jetted from the jet orifice 38 is arranged in parallel to the first straight line L. Further, the upper end 38a of the jet orifice 38 is positioned on the second straight line Lparallel to the first straight line Land coming into contact with the convex lens surface 34 at a contact point P and the lower end 38b of the jet orifice 38 is positioned on the third straight line Lparallel to the first straight line Land passing the upstream end Ein the jet direction of the fluid in the convex lens surface 34.
