An overtube that is capable of being inserted by an endoscope and a treatment tool and is elongated in a direction of a predetermined central axis, the overtube includes a tube part in which a first lumen and a second lumen are formed, the first lumen being capable of being inserted by the endoscope, and the second lumen being capable of being inserted by the treatment tool and a distal end part in which a distal opening end of the first lumen and a distal opening end of the second lumen are formed apart from each other, wherein the distal end part includes: a distal end surface that forms the distal opening end of the first lumen such that a center of the distal opening end of the first lumen is located at a position apart from the predetermined central axis, and a tapered surface that is inclined with respect to the predetermined central axis such that the tapered surface gradually approaches the first lumen from a proximal side toward a distal side in the direction of the predetermined central axis, at least part of the distal opening end of the second lumen being disposed on the tapered surface.
