PROBLEM TO BE SOLVED: To provide an adapter for treatment instrument which allows a treatment instrument to be rotated about its longitudinal axis even when the insertion part is positioned within a body, to thereby improve the operability.SOLUTION: An adapter 6 for treatment instrument comprises: a cap member 11 which is attached to a leading end of a flexible insertion part 5 having a channel, and which has a through-hole positioned at a location matching an opening of the channel； an approximately tubular engagement member which is attached within the through-hole of the cap member 11 rotatably about the axis of the through-hole and with movement in the axial direction restricted, which engages a treatment instrument 8 introduced via the channel, and which has an insertion hole engageable circumferentially with the treatment instrument； and a rotation mechanism 14 with a drive force transmission member 15 which is wound around the outer circumferential face of the engagement member and rotates the engagement member about the axis by tensile force.【課題】挿入部が体内に配置されている状態においても、処置具を長手軸回りに回転させて、操作性を向上する。【解決手段】チャネルを有する軟性の挿入部５の先端に取り付けられ、チャネルの開口に一致する位置に配置される貫通孔を有するキャップ部材１１と、該キャップ部材１１の貫通孔内に、該貫通孔の軸線回りに回転可能かつ軸線方向への移動が制限された状態で取り付けられ、チャネルを介して導入されてきた処置具８を嵌合させ、該処置具８と周方向に係合される内孔を有する略筒状の嵌合部材と、該嵌合部材の外周面に巻かれ、張力によって嵌合部材を軸線回りに回転させる駆動力伝達部材１５を備える回転機構１４とを備える処置具用アダプタ６を提供する。【選択図】図２
