An assist device includes: a connecting section communicatively connected to the channel a holding section attached to an outer side of either the insertion section of the endoscope or an operation section connected to the insertion section and a flexible section forming an insertion path through which the treatment instrument is inserted, and including an expansion-and-contraction section including a more deformable structure than any other parts in an entire length between one end and another end of the flexible section. The flexible section is connected to the connecting section at the one end and connected to the holding section at the another end.
