To provide a guide sheath whereby a plurality of insertion members can be easily guided into a body cavity.

A tubular guide sheath 1 for guiding insertion members of a medical instrument that is to be inserted into the body cavity, the guide sheath 1 comprising: an opening 11 in the base-end side which is formed in the base-end side of a guide sheath body 10 and through which a plurality of insertion members are led in; an opening 12 in the fore-end side which is formed in the fore-end side of the guide sheath body 10 and through which a plurality of insertion members are led out; and an adjusting mechanism 30 which adjusts the opening area of the opening 12 in the fore-end side.