A treatment tool for an endoscope includes a sheath a pre-curved portion which is provided at a distal portion of the sheath, and has a restoring force to restore to a curved shape in which the sheath is bent along a virtual plane including the center axis of the sheath a knife wire lumen which has a center axis at a position spaced from the virtual plane at the distal portion of the pre-curved portion, and is formed along the center axis of the sheath a first communication hole which is open in a direction outwardly away from the position of the knife wire lumen in a radial direction with respect to the center axis of the sheath, and communicates an outer circumferential surface positioned at an inward side of the curved shape of the pre-curved portion with the knife wire lumen a second communication hole which is open 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 communicates the outer circumferential surface positioned at the inward side of the curved shape of the pre-curved portion with the knife wire lumen at a more proximal position of the pre-curved portion than the first communication hole a wire-shaped cutting portion which protrudes from the first communication hole and the second communication hole and extends between the first communication hole and the second communication hole, the cutting portion being capable of incising tissues a fixing portion which is provided at a distal end portion of the cutting portion, and provided to fix the cutting portion and the knife wire lumen in a state that the fixing portion is inserted into the knife wire lumen a guide wire accommodation portion which is formed along a longitudinal axis of the sheath at a position spaced from the knife wire lumen in a circumferential direction around the center axis of the sheath, and into which a guide wire is capable of being inserted a proximal slit portion which is formed to comm
