Abstract: A conductive wire 3 is inserted into a flexible sheath 2 with a wiring portion thereof exposed at a distal end portion of the sheath. Upon the pull of the wire 3, the distal end portion is curved and the exposed wire portion is pull taut to provide a wire portion for incision. A restricting member 5 is provided within the sheath 2 to restrict the direction in which the distal end portion of the sheath 2 is curved. As viewed from the proximal end side to the distal end side of the sheath 2, the wire portion 6 is located within an angle range of 0.degree. to 90.degree. leftwise from the aforementioned direction in a manner to diagonally follow the outer surface of the sheath 2 in a counterclockwise direction to the longitudinal axis of the sheath. An affected region in the body cavity of a human being is incised with a high-frequency current carried through the conductive wire.