An assist tool attachable to an insertion section of an insertion apparatus includes a tube including a distal end portion and a proximal end portion. The tube includes a longitudinal axis extending from the distal end portion to the proximal end portion. The tube is provided with a lumen through which the insertion section is insertable. The assist tool includes a connecting mechanism provided in the tube. The connecting mechanism receives a drive force and is rotated together with the tube with respect to the insertion section inserted through the lumen. The assist tool includes a locking mechanism provided on a more distal end side than the connecting mechanism in the tube. The locking mechanism includes a convex portion or a concave portion engaged with the insertion section to restrict movement of the tube along the longitudinal axis with respect to the insertion section. Furthermore, an insertion apparatus to which the assist tool is attached and an introduction apparatus including them is provided.
