With the object of keeping the field of view of the endoscope such that the field of view of the endoscope does not fluctuate during the procedure while achieving miniaturization with a simple configuration without providing special locking means operated by the operator, The overtube (1) includes an elongated flexible insertion portion (13) having a first lumen (15) that can penetrate the endoscope (2) and a second lumen (16) that can penetrate the treatment tool, And a base end portion (14) which is fixed to the base end side of the insertion portion (13) and has a t lumen connected to the first lumen (15) and a fourth lumen connected to the second lumen (16) , The t lumen curves in a direction away from the fourth lumen toward the proximal side and the fourth lumen extends substantially linearly along the longitudinal axis of the second lumen (16). 操作者によって操作される特別な係止手段を設けることなく、簡易な構成で小型化を図りつつ、処置中に内視鏡の視野が変動しないように保持することを目的として、本発明に係るオーバーチューブ（１）は、内視鏡（２）を貫通可能な第１ルーメン（１５）と、処置具を貫通可能な第２ルーメン（１６）とを備える細長い軟性の挿入部（１３）と、挿入部（１３）の基端側に固定され、第１ルーメン（１５）に接続する第３ルーメンと、第２ルーメン（１６）に接続する第４ルーメンとを備える基端部（１４）とを備え、第３ルーメンが、基端側に向かって第４ルーメンから離れる方向に湾曲し、第４ルーメンが、第２ルーメン（１６）の長手軸に沿って略直線状に延びている。
