In a high-frequency knife, a conduit line and an opening of an opening hole on a distal end side communicate with each other so that a fluid is allowed to flow through a gap between an outer peripheral surface of a rod-shaped electrode and an inner peripheral surface of the opening hole. The guide hole is smaller than the opening of the opening hole and is located further toward a radial inner side than an outer edge of the opening and further toward a radial outer side than the rod-shaped electrode. The fluid is released from the opening to the outside of the sheath over the whole circumference of the rod-shaped electrode on the radial outer side in a state where an electrode portion is made to protrude toward a distal end portion of the sheath so that a larger-diameter portion is separated from a supporting member. The fluid is injected to the front of the larger-diameter portion through the guide hole as the opening hole and the guide hole communicate with each other in a pull-back state where the electrode portion is pulled back toward a proximal end portion of the sheath so that the larger-diameter portion abuts against the supporting member.
