A medical sheath is a medical sheath to be used in combination with a medical device including an insertion portion that can be inserted into a body cavity of a subject via a trocar when treatment of an examined region present in the body cavity of the subject is performed using the medical device, the medical sheath including: a sheath body formed in a shape that can be inserted to and removed from a through hole through which the insertion portion is inserted in the trocar a channel provided in the sheath body, the channel being a conduit formed so as to have a shape that enables insertion and removal of the insertion portion and an optical transmission section provided in the sheath body, the optical transmission section being provided for guiding at least one of illuminating light to be applied to the examined region and return light emitted from the examined region upon application of the illuminating light.
