This high-frequency treatment instrument comprises: a sheath (2) formed of a flexible material; an electrode member (3) disposed inside the sheath (2); a distal-end member (6) having a through-hole (6a) through which the electrode member (3) passes; and a liquid supply means (5) supplying liquid towards the front side via a flow path (2a) in the sheath (2), wherein the electrode member (3) includes a rod-shaped electrode portion (3a) movably passing through the through-hole (6a), and a large-diameter portion (3b) provided at the distal end thereof to have an outer diameter larger than the diameter of the through-hole (6a), the large-diameter portion (3B) includes an electrode hole (3d) passing therethrough, between the distal-end member (6) and the large-diameter portion (3b), there is provided a buffer member (10) that seals the periphery of a space (A) between the distal-end member (6) and the large-diameter portion (3).