A treatment-instrument insertion aid (10) for aiding insertion of a treatment instrument into a body includes an inner tube (20) having flexibility into which the treatment instrument is insertable, and an outer tube (30) having flexibility into which the inner tube (20) is insertable. Both end portions (20B) of the inner tube (20) are formed of a hard material, and an intermediate portion (20A) between both the end portions (20B) is formed of a soft material.