A treatment instrument insertion tool includes a tubular treatment instrument guide portion, configured to guide a treatment instrument into an insertion portion of an insertion device via a treatment instrument insertion port, a guide portion body freely attachably/detachably provided in a region of the insertion portion located outside a subject when the insertion portion is inserted into the subject, and a film member including a hole that allows insertion of the treatment instrument, water-tightly engaging with the treatment instrument when the treatment instrument is inserted into the hole, and being formed of a material freely expandable/contractible and restricting permeation of a liquid.
