A high-frequency treatment instrument having a sheath； an elongated extension member； a narrow portion provided on an inner surface of the sheath； and a pressing member which allows the elongated extension member to advance and retract only in a part of the sheath. The elongated extension member is advanced to form a first treatment electrode extending along a central axial line of the sheath and the elongated extension member is further advanced to form a second treatment electrode which intersects with the central axial line of the sheath.
