high-frequency knife (1) has a sheath (10) and the electrode portion (25). Electrode portion (25) has a rod-like electrode (26), a cylindrical electrode surrounding the rod-shaped electrode (26) (28), and an auxiliary electrode extending from the cylindrical electrode (28) in the radial direction (29). At the tip of the rod electrode (26), electrically insulating large-diameter portion (27) is provided. Large-diameter portion (27) has a guide hole extending therethrough (31). The sheath conduit (10) (10a) fluid supplied through the (physiological saline) is injected in front of the large-diameter portion (27) through the guide hole (31). When the high-frequency knife (1) is cut tissue, sometimes burnt tissue (T1) from sticking to the outer peripheral surface of the cylindrical electrode (28) (28 b). Since the guide hole (31) is provided on the projecting portion of the large-diameter portion (27) (30), not easily blocked by tissue sticks to the outer peripheral surface (28 b) (T1).高周波ナイフ(1)はシース(10)及び電極部(25)を有する。電極部(25)は、棒状電極(26)と、棒状電極(26)を囲う筒状電極(28)と、筒状電極(28)から径方向に延びる補助電極(29)とを有する。棒状電極(26)の先端部には、電気絶縁性の大径部(27)が設けられる。大径部(27)はこれを貫通するガイド孔(31)を有する。シース(10)の管路(10a)を通って供給される流体(生理食塩水)はガイド孔(31)を通って大径部(27)の前方に噴射される。高周波ナイフ(1)が組織を切開するとき、焦げた組織(T1)が筒状電極(28)の外周面(28b)に貼り付くことがある。ガイド孔(31)は大径部(27)の突出部(30)上に設けられているから、外周面(28b)に貼り付いた組織(T1)によって塞がれにくい。
