An intra-body-cavity insertion instrument fastener includes an insertion section that is inserted into a body cavity and is held, partially exposed to outside of a human body, and a fastening section that fixes, to a predetermined position, a position of a predetermined point of an intra-body-cavity insertion instrument inserted into the body cavity from an insertion hole different from an insertion hole from which the insertion section is inserted into the body cavity. The fastening section is formed at a top end of the insertion section.