The endoscopic treatment instrument includes a sheath formed with a lumen extending in a longitudinal direction and having an inner diameter through which a guide wire can be inserted, and a sheath disposed at a proximal end of the sheath and having a proximal end portion And an operation part having a substantially tubular shape having an inner diameter that allows insertion thereof and having a port opened so as to communicate with an outer peripheral surface from an inner peripheral surface, wherein the sheath is formed so that the sheath extends in the longitudinal axis direction A slit portion formed on an outer peripheral surface of the sheath so as to communicate the lumen and the outside of the sheath along a slit portion formed so that an opening width in the circumferential direction of the sheath is smaller than the inner diameter of the lumen And an inlet portion having a width greater than the opening width of the slit portion and communicating with the lumen and opening to the outer peripheral surface of the proximal end portion of the sheath, An opening edge portion and said paw The inner opening edge portion forming the contour on the inner peripheral surface side of the inlet portion when projecting the inlet portion from the direction along the straight line orthogonal to the central axis of the lumen and the center axis of the sheath to the inner peripheral surface of the port The sheath is fixed to the operating portion such that the opening edge portion is positioned substantially coincident with the inner opening edge portion or outside the inner opening edge portion.本内視鏡用処置具は、長手軸方向に延びてガイドワイヤを挿通可能な内径を有するルーメンが形成されたシースと、前記シースの近位端に配されて前記シースの前記近位端部分が挿入可能な内径を有して略筒状に形成され、内周面から外周面に連通するように開口したポートを有する操作部と、を備え、前記シースは、前記シースの前記長手軸方向に沿って前記ルーメンと前記シースの外部とを連通するように前記シースの外周面に形成され、前記シースの周方向における開口幅が前記ルーメンの前記内径よりも小さくなるように形成されたスリット部と、前記スリット部の前記開口幅よりも大きい幅を有して前記ルーメンと連通して前記シースの前記近位端部分の外周面に開口された入口部と、を有し、前記入口部の開口縁部及び前記ポートの内周面側の輪郭を形成する内側開口縁部を、前記ルーメンの中心軸および前記シースの中心軸に直交する直線
