Abstract: A high-frequency incision device has a flexible sheath adapted to be inserted into the patient's body through the insertion channel of an endoscope. An end member is fixed to the distal end of the sheath and has a through hole extending in the axial direction of the sheath. The hole has a small diameter section on the side of the distal end of the sheath, a large diameter section, and a shoulder defined between the sections. A knife for piercing a desired portion is inserted through the hole to be capable of being project from and retreated into the sheath. The knife has a stopper portion integrally formed therewith and having a width greater than the small diameter section and smaller than the large diameter section. The stopper portion restricts the extent of projection of the knife cooperating with the shoulder. The knife is connected to a high-frequency power source through an operating wire.