A first convex portion is arranged at an outer peripheral portion of a first support portion constituting a distal end portion of an insertion portion, and a protruding amount of the first convex portion is set such that an outer peripheral length of the distal end portion including the first convex portion is smaller than an outer peripheral length determined based on a distal end outer diameter as an effective outer diameter of the distal end portion, 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.