A treatment instrument insertion tool includes a main body section, an attachment portion configured to attach the main body section to an insertion section, a guide section that is connectable to a treatment instrument insertion port of an endoscope, and that is configured to guide a treatment instrument extending from the main body section into the insertion section through the treatment instrument insertion port, and an operation member that is provided on the main body section, that can fix the treatment instrument that is inserted in the main body section, by being at least partially elastically deformed, and that is configured to perform an operation of moving the treatment instrument forward or backward along a direction of an axis of the treatment instrument and an operation of rotating the treatment instrument around the axis in a fixed state in which the treatment instrument is fixed.
