A high-frequency treatment instrument comprising: a sheath; an elongated extension member which is disposed in the sheath and is configured to be freely extendable and retractable, the elongated extension member comprising: a first linear portion disposed along the sheath, a first bent portion formed at a distal end of the first linear portion, a second linear portion extending from the first bent portion, and a second bent portion formed halfway along the second linear portion, wherein when the first linear portion and the second linear portion extend toward the distal end of the sheath and thus the first bent portion and the second bent portion protrude from the sheath, a treatment electrode intersecting with a center axial line of the sheath is configured to be formed between the first bent portion and the second bent portion, characterized by a narrow tube portion provided on an inner surface of the sheath; a control portion disposed at a position that is closer to a proximal end of the sheath than the second bent portion; and pressing members (66A, 66B) disposed halfway along the first linear portion and at a distal end of the second linear portion, respectively, wherein the control portion is a tube-shaped diameter-enlarged portion and is disposed between the pressing members (66A, 66B), the narrow tube portion is configured to control movement of the control portion toward a distal end side of the sheath, when the second bent portion is protruded from the sheath by a predetermined length while the first bent portion is still remained inside of the sheath, the control portion is configured to come in contact with the narrow tube portion, and a portion of the elongated extension member which protrudes from the sheath is configured to become the treatment electrode which extends along a center axial line of the sheath, and the control portion is configured to control a protruding amount of the first linear portion and the second linear portion from the distal end of the sheath by contacting the narrow tube portion. a sheath; an elongated extension member which is disposed in the sheath and is configured to be freely extendable and retractable, the elongated extension member comprising: a first linear portion disposed along the sheath, a first bent portion formed at a distal end of the first linear portion, a second linear portion extending from the first bent portion, and a second bent portion formed halfway along the second linear portion, wherein a first linear portion disposed along the sheath, a first bent portion formed at a distal end of the first linear portion, a second linear portion extending from the first bent portion, and a second bent portion formed halfway along the second linear portion, wherein when the first linear portion and the second linear portion extend toward the distal end of the sheath and thus the first bent portion and the second bent portion protrude from the sheath, a treatment electrode intersecting with a center axial line of the sheath is configured to be formed between the first bent portion and the second bent portion, characterized by a narrow tube portion provided on an inner surface of the sheath; a control portion disposed at a position that is closer to a proximal end of the sheath than the second bent portion; and pressing members (66A, 66B) disposed halfway along the first linear portion and at a distal end of the second linear portion, respectively, wherein the control portion is a tube-shaped diameter-enlarged portion and is disposed between the pressing members (66A, 66B), the narrow tube portion is configured to control movement of the control portion toward a distal end side of the sheath, when the second bent portion is protruded from the sheath by a predetermined length while the first bent portion is still remained inside of the sheath, the control portion is configured to come in contact with the narrow tube portion, and a portion of the elongated extension member which protrudes from the sheath is configured to become the treatment electrode which extends along a center axial line of the sheath, and the control portion is configured to control a protruding amount of the first linear portion and the second linear portion from the distal end of the sheath by contacting the narrow tube portion.