A high-frequency treatment instrument for performing a high-frequency treatment on a pathological lesion portion includes an elongated flexible sheath 2, a pair of arm portions 3A and 3B which is disposed in the sheath 2 so as to freely extend and retract, which is in a closed state in the sheath, and which is opened in a direction moving away from a center axial line of the sheath 2 when it protrudes from the distal end of the sheath 2, a linear treatment electrode which is connected to the distal end sides of the pair of arm portions and which is stretched between the arm portions when the pair of arm portions are opened, an operating wire (elongated extension member) which is disposed in the sheath 2 so as to freely extend and retract and the distal end of which is connected to the treatment electrode, and an operating portion, to which the proximal ends of the sheath and the operating wire are connected, and which operates the operating wire to extend and retract relative to the sheath 2. According to the invention, it is possible to facilitate a procedure by enabling a treatment on a target tissue without performing a curving operation of an endoscope.
