The high-frequency knife has the electrode portion, the electrode portion includes a rod-shaped electrode which extends in the axis direction and is connected to the operating wire, a larger-diameter portion which has electric insulation, and has a larger external diameter than an external diameter of the rod-shaped electrode, a tubular electrode which is formed in a tubular, extends toward a proximal end side so as to surround the rod-shaped electrode from a proximal end surface of the larger-diameter portion, and is connected to the rod-shaped electrode, and an auxiliary electrodes which extend in a radial direction from an outer peripheral surface of the tubular electrode. A guide hole is provided in the larger-diameter portion. The guide hole extends in the aixs direstion and protrudes the larger-diameter portion, is capable of jetting the fluid that is supplied through the couduit line of the sheath from a front of the large-diameter portion, and is positioned at an outer side of the radial direction of the tubular electrode when being viewed in the axis direction. A distance increasing portion, which increases a creeping distance along an external surface of the larger-diameter portion between an outer peripheral surface of the tubular electrode exposed to an outside and an opening of a proximal end side of the guide hole, is provided in the proximal end surface of the larger-diameter portion.
