High-frequency knife

A high-frequency knife conduit line and opening hole opening communicate allowing fluid to flow between a rod-shaped electrode outer peripheral surface and the opening hole inner peripheral surface. The guide hole is smaller than the opening and is further toward a radial inner side than the opening outer edge 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 while an electrode portion protrudes toward the sheath distal end portion separating a larger-diameter portion from a supporting member. The fluid is injected to the larger-diameter portion front through the guide hole as the opening hole and the guide hole communicate while the electrode portion is pulled back toward the sheath proximal end portion so that the larger-diameter portion abuts against the supporting member.