A treatment-instrument insertion aid (10) for aiding in the insertion of a treatment instrument into a body is equipped with: a flexible outer tube (30) that has a plurality of guide parts (31) extending in the axial direction from the tip end side toward the base end side, and positioned on the inner-circumferential surface thereof at a distance from one another in the circumferential direction； a plurality of flexible rail members (40) that each have a guidable part (41) to be engaged by a guide part (31), and extend in the axial direction from the tip end side toward the base end side； and a flexible inner tube (20) having an engageable part (21) to be engaged by an engaging part (50) formed between the inner-circumferential surface of the outer tube (30) and an adjacent rail member (40). Furthermore, the rail members (40) are affixed to rigid sections (30B) positioned farthest toward the tip end side of the outer tube (30).本發明提供一種可將外管良好地彎曲之處置具插入輔助具。可輔助處置具朝體內插入之處置具插入輔助具(10)，具備：外管(30)，其具有可撓性、而且具有在內周面於圓周方向相隔之自前端側以至基端側於軸向延伸之複數個引導部(31)；複數個軌道部件(40)，其具有可撓性、而且分別具有自前端側以至基端側於軸向延伸之卡合於引導部(31)之被引導部(41)；及內管(20)，其具有可撓性、而且具有被卡合部(21)，該被卡合部(21)係卡合於形成在外管(30)之內周面與相鄰之軌道部件(40)之間之卡合部(50)。軌道部件(40)分別固定於外管(30)之最前端側之硬質部(30B)。
