The catheter (1), which is a treatment instrument for an endoscope, includes a port (23), a port (23) connected to the proximal region (4) and the proximal region (4) inserted into the port (23). A sheath (2) having a distal region extending outward from the sheath (2), a distal end in the distal region of the sheath (2), and a proximal end in the proximal region (4) of the sheath (2), A flap (7) extending in the longitudinal axis direction of the sheath (2) from the distal end to the proximal end of (2), and between the proximal end of the flap (7) and the proximal end of the sheath (2), the sheath (2) Is provided between the notch surface (11) having a shape in which the flap (7) is notched along the longitudinal axis, and the inner surface of the notch surface (11) and the port (23), and the length of the sheath (2). There is a locking surface (48) that can contact the notch surface (11) so as to limit the movement of the notch surface (11) in the rotational direction about the axis. A rolling and preventing member (40), the.内視鏡用処置具であるカテーテル（１）は、ポート（２３）と、ポート（２３）の内部に挿入された近位領域（４）と近位領域（４）から連なってポート（２３）の外部へ延びた遠位領域とを有するシース（２）と、シース（２）の遠位領域に先端を有し、シース（２）の近位領域（４）に基端を有し、シース（２）の先端から基端までシース（２）の長手軸方向に延びたフラップ（７）と、フラップ（７）の基端とシース（２）の基端との間で、シース（２）の長手軸に沿ってフラップ（７）が切り欠かれた形状をなす切欠面（１１）と、切欠面（１１）とポート（２３）の内面との間に設けられ、シース（２）の長手軸周りの回転方向へ切欠面（１１）が移動するのを制限するように切欠面（１１）に接触可能な係止面（４８）を有する回転防止部材（４０）と、を有する。
