Abstract: An apparatus for optically observing a body cavity in which the view field is made more clear by eliminating mucus in the body cavity. The apparatus includes a tubular fluid passage forming member which surrounds an insert part and which is inserted into the body cavity. A space between the fluid passage forming member and the outer face of the insert part is divided into a fluid supply space and discharge space. The fluid supply space is externally supplied with fluid such as clean water and the mucus in the body cavity is discharged to the outside of the body via the fluid discharge space. The fluid passage forming member is made of economical material such as plastics so that it may be disposable, reducing the risk of infecting a patient with a virus.