A treatment-instrument insertion aid (10) for aiding insertion of a treatment-instrument into a body includes an outer tube (30) having flexibility including a plurality of guiding portions (31) circumferentially spaced apart on an inner peripheral surface and extend in an axial direction from a distal end to a proximal end, a plurality of rail members (40) having flexibility, extend in an axial direction from a distal end to a proximal end and include guided portions (41) to be engaged with the guiding portions (31) respectively, and an inner tube (20) having flexibility and includes an engaged portion (21) to be engaged with an engaging portion (50) formed between the inner peripheral surface of the outer tube (30) and the adjacent rail members (40). The rail members (40) are fixed to a hard portion (30B) nearest to the distal end side of the outer tube (30).
