An object of the present invention is to provide means capable of steering a treatment instrument together with an endoscope with good operability in endoscopic surgery. The endoscope overtube of the present invention includes a first insertion passage into which an endoscope is inserted, wherein a wall constituting the first insertion passage includes a second insertion passage into which a treatment instrument is inserted, and a major axis direction of the second insertion passage is different from a major axis direction of the first insertion passage at a distal end of the second insertion passage. Since the treatment instrument insertion passage of the overtube does not extend in a direction along the major axis of the overtube, the treatment instrument is protruded from the distal end of the overtube in a direction different from that of the center of a surgical field, and extends outward, away from a direction along the viewing axis of the overtube. For example, when a grasping forceps is inserted into the insertion passage of the overtube, by adjusting the extent of protrusion of the grasping forceps, it is possible to grip a tissue under an appropriate tension and perform a treatment such as removal with a treatment instrument such as an electrocautery inserted from the treatment instrument channel of the endoscope.
