This high-frequency treatment instrument includes a sheath (2) made of a flexible material, an electrode member (3) disposed in the sheath (2) so as to be able to advance and retreat, and a through hole (6a) that penetrates the electrode member (3). And a liquid feeding means (5) for supplying liquid to the front in the longitudinal axis direction of the sheath (2) via the flow path (2a) in the sheath (2), and an electrode member ( 3) is made of an insulating material having a rod-shaped electrode portion (3a) that is movably penetrated into the through-hole (6a) and an outer diameter that is provided at the tip of the rod-shaped electrode portion (3a) and larger than the diameter of the through-hole (6a). A large-diameter portion (3b), the large-diameter portion (3b) includes an electrode hole (3d) penetrating in the longitudinal axis direction, and an electrode member (between the tip member (6) and the large-diameter portion (3b)) 3) A tip member that is connected to the through hole (6a) and the electrode hole (3d) when it is moved to the proximal side to the maximum Around the space A between 6) and a large diameter portion (3b) was sealed over the entire periphery, provided with a cushioning member that is elastically deformed in the longitudinal axis direction (10). この高周波処置具は、可撓性材料からなるシース（２）と、シース（２）内に進退可能に配置された電極部材（３）と、電極部材（３）を貫通させる貫通孔（６ａ）を有する先端部材（６）と、シース（２）内の流路（２ａ）を介してシース（２）の長手軸方向前方に液体を供給する送液手段（５）とを備え、電極部材（３）が、貫通孔（６ａ）内に移動可能に貫通させられる棒状電極部（３ａ）と、その先端に設けられ貫通孔（６ａ）の口径よりも大きな外径寸法を有し絶縁材料からなる大径部（３ｂ）とを備え、大径部（３ｂ）が長手軸方向に貫通する電極孔（３ｄ）を備え、先端部材（６）と大径部（３ｂ）との間に電極部材（３）が最大限に基端側に移動させられたときに、貫通孔（６ａ）および電極孔（３ｄ）に接続する先端部材（６）と大径部（３ｂ）との間の空間Ａの周囲を全周にわたって密封し、長手軸方向に弾性変形させられる緩衝部材（１０）を備える。
