There is provided an overtube that can suppress contact of an opening edge portion of a ventilation hole of the overtube with a lumen inner wall surface. The overtube includes an overtube body that has a distal end and a proximal end and allows an insertion part of an endoscope, which is to be inserted into a lumen, to be inserted therein, a balloon that is mounted on an outer circumferential surface of the overtube body, a protruding part that is formed on the outer circumferential surface, and an ventilation hole that is formed in the outer circumferential surface and allows the outer circumferential surface and an inner circumferential surface of the overtube body to communicate with each other. On the outer circumferential surface, one of the protruding part or the ventilation hole is formed in a peripheral portion of the other.