A first convex portion 60 is arranged at an outer peripheral portion of a first support portion 32 constituting a distal end portion 5 of an insertion portion 2, and a protruding amount of the first convex portion 60 is set such that an outer peripheral length L 1 of the distal end portion 5 including the first convex portion 60 is smaller than an outer peripheral length L0 determined based on a distal end outer diameter D0 as an effective outer diameter of the distal end portion 5, to thereby ensure durability without an unnecessary increase in a size of the distal end portion and improve insertability of the distal end portion into a subject.
