A high-frequency knife comprising: a sheath which has electric insulation and flexibility, has a conduit line formed therein, and extends in an axis direction,an operating wire which is inserted through the conduit line so as to be capable of advancing and retracting in the axis direction; and an electrode portion which is provided at a distal end portion of the operating wire, wherein the electrode portion includes: a first electrode which is rod-shaped, extends in the axis direction, and has a proximal end portion electrically connected to the operating wire; a second electrode which is electrically connected to a distal end of the first electrode and is formed so as to surround the distal end of the first electrode; and a larger-diameter portion which is provided at the distal end portion of the first electrode, has electric insulation, and has a larger external diameter than an external diameter of the first electrode, wherein a protruding portion is provided on a proximal end surface of the larger-diameter portion so as to protrude toward a proximal end portion of the first electrode, the second electrode is formed so that a proximal end surface of the second electrode coincides with a proximal end surface of the protruding portion, characterised in that the larger-diameter portion has a guide through hole that extends in the axis direction, and in that an opening that communicates with the guide hole is formed on the proximal end surface of the protruding portion. a sheath which has electric insulation and flexibility, has a conduit line formed therein, and extends in an axis direction,an operating wire which is inserted through the conduit line so as to be capable of advancing and retracting in the axis direction; and an electrode portion which is provided at a distal end portion of the operating wire, wherein the electrode portion includes: a first electrode which is rod-shaped, extends in the axis direction, and has a proximal end portion electrically connected to the operating wire; a second electrode which is electrically connected to a distal end of the first electrode and is formed so as to surround the distal end of the first electrode; and a larger-diameter portion which is provided at the distal end portion of the first electrode, has electric insulation, and has a larger external diameter than an external diameter of the first electrode, wherein a protruding portion is provided on a proximal end surface of the larger-diameter portion so as to protrude toward a proximal end portion of the first electrode, the second electrode is formed so that a proximal end surface of the second electrode coincides with a proximal end surface of the protruding portion, characterised in that the larger-diameter portion has a guide through hole that extends in the axis direction, and in that an opening that communicates with the guide hole is formed on the proximal end surface of the protruding portion. a first electrode which is rod-shaped, extends in the axis direction, and has a proximal end portion electrically connected to the operating wire; a second electrode which is electrically connected to a distal end of the first electrode and is formed so as to surround the distal end of the first electrode; and a larger-diameter portion which is provided at the distal end portion of the first electrode, has electric insulation, and has a larger external diameter than an external diameter of the first electrode, wherein a protruding portion is provided on a proximal end surface of the larger-diameter portion so as to protrude toward a proximal end portion of the first electrode, the second electrode is formed so that a proximal end surface of the second electrode coincides with a proximal end surface of the protruding portion, characterised in that the larger-diameter portion has a guide through hole that extends in the axis direction, and in that an opening that communicates with the guide hole is formed on the proximal end surface of the protruding portion.