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, the medical sheath including: a sheath body formed in a shape that can be inserted to and removed from a through hole through in the trocar; a channel provided in the sheath body, the channel having a shape that enables insertion and removal of the insertion portion; a first optical transmission section provided in the sheath body, the first optical transmission section being provided for guiding illuminating light; and a second optical transmission section that is provided in the sheath body and is separate from the first optical transmission section, the second optical transmission section being provided for guiding return light generated upon application of the illuminating light from the first optical transmission section.