An insertion assist device in an insertion apparatus, the insertion assist device provided outside an insertion body including a first flexible tube, a second flexible tube different from the first flexible tube, and a base including a small-diameter portion coupled to the first flexible tube and a large-diameter portion coupled to the second flexible tube and being larger in diameter than the small-diameter portion and in which a rotation force transmission portion disposed and rotated when rotation force of the rotation force transmission unit is transmitted to the rotation force transmission portion, includes: an inner cylindrical body meshing with the rotation force transmission portion and rotatable around the base while the rotation force transmission portion is disposed in the base and a tube supporting the inner cylindrical body and configured to cover at least part of the first flexible tube while the inner cylindrical body is in mesh with the rotation force transmission portion.
