An endoscope system has an endoscope; an attachment for the endoscope, having a non-circular irregularly shaped hole that communicates with an opening of a channel of the endoscope and attached to a forward end of an insertion section of the endoscope; and a treatment instrument for the endoscope, having a flexible tube and a treatment section. An extension/retraction section having a non-circular cross section corresponding to the shape of an opening of the irregularly shaped holes is provided at a forward end of the flexible tube, and the irregularly shaped hole restricts rotation of the flexible tube. Further, the treatment section has directivity about its axis relative to the flexible tube and its rotation is restricted.