A treatment tool for an endoscope, comprising: a sheath which is extended along a longitudinal axis; a pre-curved portion which is disposed at a distal portion of the sheath, and has a restoring force to restore the pre-curved portion to a curved shape in which a center axis of the sheath is curved in a predetermined virtual plane; a knife wire lumen which is formed along the longitudinal axis of the sheath and has a center axis positioned spaced from the virtual plane at a distal end portion of the pre-curved portion; a first communication hole which is opened in a direction outwardly away from a position of the knife wire lumen in a radial direction with respect to the center axis of the sheath at an inward side of the curved shape and spaced from the virtual plane at the distal end portion of the pre-curved portion, and communicates an outer circumferential surface positioned at the inward side of the curved shape of the pre-curved shape with the knife wire lumen; a second communication hole which is opened in a direction outwardly away from the position of the knife wire lumen in the radial direction with respect to the center axis of the sheath and spaced from the virtual plane, and communicates the outer circumferential surface positioned at the inward side of the curved shape with the knife wire lumen at a more proximal position of the pre-curved portion than the first communication hole; a knife wire having a wire-shaped cutting portion which includes a bending portion; a fixing portion which fixes a distal end portion of the knife wire inside the knife wire lumen; and a guide wire lumen which is formed inside the sheath, wherein the bending portion has a bent shape formed by bending the knife wire in a direction away from both the virtual plane and an outer surface of the inward side of the curved shape from a direction in which the first communication hole is opened, and in a direction parallel with a tangential line of the outer circumferential surface of the sheath, the cutting portion is protruded from the first communication hole and the second communication hole, the cutting portion is configured to be capable of incising tissues, and the cutting portion is extended between the first communication hole and the second communication hole when the pre-curved portion is curved to the curved shape; and wherein a slit is formed to be opened at the circumferential surface of the sheath, the slit communicating the guidewire lumen with an outside of the sheath, and the slit being formed to be extended along the center axis of the sheath. a sheath which is extended along a longitudinal axis; a pre-curved portion which is disposed at a distal portion of the sheath, and has a restoring force to restore the pre-curved portion to a curved shape in which a center axis of the sheath is curved in a predetermined virtual plane; a knife wire lumen which is formed along the longitudinal axis of the sheath and has a center axis positioned spaced from the virtual plane at a distal end portion of the pre-curved portion; a first communication hole which is opened in a direction outwardly away from a position of the knife wire lumen in a radial direction with respect to the center axis of the sheath at an inward side of the curved shape and spaced from the virtual plane at the distal end portion of the pre-curved portion, and communicates an outer circumferential surface positioned at the inward side of the curved shape of the pre-curved shape with the knife wire lumen; a second communication hole which is opened in a direction outwardly away from the position of the knife wire lumen in the radial direction with respect to the center axis of the sheath and spaced from the virtual plane, and communicates the outer circumferential surface positioned at the inward side of the curved shape with the knife wire lumen at a more proximal position of the pre-curved portion than the first communication hole; a knife wire having a wire-shaped cutting portion which includes a bending portion; a fixing portion which fixes a distal end portion of the knife wire inside the knife wire lumen; and a guide wire lumen which is formed inside the sheath, wherein the bending portion has a bent shape formed by bending the knife wire in a direction away from both the virtual plane and an outer surface of the inward side of the curved shape from a direction in which the first communication hole is opened, and in a direction parallel with a tangential line of the outer circumferential surface of the sheath, the cutting portion is protruded from the first communication hole and the second communication hole, the cutting portion is configured to be capable of incising tissues, and the cutting portion is extended between the first communication hole and the second communication hole when the pre-curved portion is curved to the curved shape; and wherein a slit is formed to be opened at the circumferential surface of the sheath, the slit communicating the guidewire lumen with an outside of the sheath, and the slit being formed to be extended along the center axis of the sheath.