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