An endoscopic attachment comprising: a cover portion that is adapted to be mounted on a distal end of an inserted portion of an endoscope so as to be capable of covering a portion of an opening portion of a channel formed in the inserted portion of the endoscope; and an endoscope mounting portion that is adapted to be fixed to the inserted portion, wherein a deformed hole is formed by the cover portion, the deformed hole being adapted to be in the opening portion of the channel, the cover portion includes a tapered induction portion which has a same circular cross section as the opening of the channel to induce an endoscopic treatment instrument which has a non-circular cross-section corresponding to the deformed hole from the channel to the deformed hole, the induction portion passes through an opening of an outer surface of the cover portion and an opening of an inner surface of the cover portion, and the deformed hole is configured to have a shape capable of being fitted to an endoscopic treatment instrument, which projects from the opening of the channel and has a non-circular cross section corresponding to the deformed hole, characterized in that: the deformed hole is a non-circular deformed hole, the rotation of the endoscopic treatment instrument is restricted by a fitting of the non-circular cross-section of the endoscopic treatment instrument with the non-circular deformed hole, and the endoscopic treatment instrument freely moves in and out of the opening portion of the channel in a predetermined direction while the rotation of the endoscopic treatment instrument is restricted. a cover portion that is adapted to be mounted on a distal end of an inserted portion of an endoscope so as to be capable of covering a portion of an opening portion of a channel formed in the inserted portion of the endoscope; and an endoscope mounting portion that is adapted to be fixed to the inserted portion, wherein a deformed hole is formed by the cover portion, the deformed hole being adapted to be in the opening portion of the channel, the cover portion includes a tapered induction portion which has a same circular cross section as the opening of the channel to induce an endoscopic treatment instrument which has a non-circular cross-section corresponding to the deformed hole from the channel to the deformed hole, the induction portion passes through an opening of an outer surface of the cover portion and an opening of an inner surface of the cover portion, and the deformed hole is configured to have a shape capable of being fitted to an endoscopic treatment instrument, which projects from the opening of the channel and has a non-circular cross section corresponding to the deformed hole, characterized in that: the deformed hole is a non-circular deformed hole, the rotation of the endoscopic treatment instrument is restricted by a fitting of the non-circular cross-section of the endoscopic treatment instrument with the non-circular deformed hole, and the endoscopic treatment instrument freely moves in and out of the opening portion of the channel in a predetermined direction while the rotation of the endoscopic treatment instrument is restricted. the deformed hole is a non-circular deformed hole, the rotation of the endoscopic treatment instrument is restricted by a fitting of the non-circular cross-section of the endoscopic treatment instrument with the non-circular deformed hole, and the endoscopic treatment instrument freely moves in and out of the opening portion of the channel in a predetermined direction while the rotation of the endoscopic treatment instrument is restricted.