An intracorporeal insertion instrument 11 includes an elongated two guide wires 25, 26 each having a distal end portion whose cross-sectional shape is non-circular, wherein a bending habit is applied to the distal end portion, and a sheath main body 22 provided with a plurality of lumens 23, 24 formed such that the guide wires 25, 26 are inserted from a plurality of proximal-end openings and the respective guide wires can be protruded from respective distal-end openings, the respective distal-end openings being formed such that the respective guide wires are prevented from rotating around axes of the guide wires in the distal end openings, and the distal end portions of the guide wires 25, 26 are separated from each other along protruding directions.
