A soft sheath and a pair of opening and closing elements which protrudes from and retracts into the soft sheath and is capable of opening and closing are provided in an insertion portion capable of being inserted through a treatment device insertion channel of an endoscope, a slender arm and a high-frequency surgical blade disposed at the distal end of the arm are formed in each opening and closing element, and a high-frequency treatment portion which converges a high-frequency current is provided on the blade.