Provided is a guide sheath that is prevented from coming out of a body cavity so as to allow for stable operation. A tubular guide sheath that guides an insertion section of a medical device to be inserted into a body cavity includes a base-end opening that is provided at a base end of the guide sheath and into which the insertion section is inserted; a distal-end opening that is provided at a distal end of the guide sheath; a flexible bent section that is provided at the distal end of the guide sheath and is elastically bent; and an exit opening that is provided in an outer side surface of the bent section and from which the insertion section extends out.