An intracorporeal insertion instrument configured to be inserted into a bile duct in a body when protruding from a channel of an endoscope, the intracorporeal insertion instrument comprising: a first elongated and flexible guide wire having an insertion axis, and a distal end portion whose cross-sectional shape is a non-circular shape and to which a first bending habit is applied in advance to be bent in a predetermined direction; a second elongated and flexible guide wire having an insertion axis, and a distal end portion whose cross-sectional shape is a non-circular shape and to which a second bending habit is applied in advance to be bent in a predetermined direction; a sheath main body provided with a first lumen and a second lumen through which the first guide wire and the second guide wire are insertable, respectively, the first and second lumens (23, 24) having cross-sectional shapes complementary to cross-sectional shapes of the first and second guide wires (25, 26), respectively, such that the first and second guide wires (25, 26) are prevented from rotating around the insertion axes; a distal end portion provided on a distal end side of the sheath main body and forming a first distal-end opening of the first lumen and a second distal-end opening of the second lumen; and a proximal end portion provided on a proximal end side of the sheath main body and forming a first proximal-end opening of the first lumen which is larger than the first distal-end opening and a second proximal-end opening of the second lumen which is larger than the second distal- end opening, the proximal end portion being configured for inserting the first guide wire and the second guide wire such that the first guide wire and the second guide wire are capable of protruding from the first distal- end opening and the second distal-end opening, respectively, wherein, when the first guide wire is protruded from the first distal-end opening and the second guide wire is protruded from the second distal- end opening, the first guide wire and the second guide wire are protruded to bend in directions different from each other by the first bending habit of the first guide wire and the second bending habit of the second guide wire. a first elongated and flexible guide wire having an insertion axis, and a distal end portion whose cross-sectional shape is a non-circular shape and to which a first bending habit is applied in advance to be bent in a predetermined direction; a second elongated and flexible guide wire having an insertion axis, and a distal end portion whose cross-sectional shape is a non-circular shape and to which a second bending habit is applied in advance to be bent in a predetermined direction; a sheath main body provided with a first lumen and a second lumen through which the first guide wire and the second guide wire are insertable, respectively, the first and second lumens (23, 24) having cross-sectional shapes complementary to cross-sectional shapes of the first and second guide wires (25, 26), respectively, such that the first and second guide wires (25, 26) are prevented from rotating around the insertion axes; a distal end portion provided on a distal end side of the sheath main body and forming a first distal-end opening of the first lumen and a second distal-end opening of the second lumen; and a proximal end portion provided on a proximal end side of the sheath main body and forming a first proximal-end opening of the first lumen which is larger than the first distal-end opening and a second proximal-end opening of the second lumen which is larger than the second distal- end opening, the proximal end portion being configured for inserting the first guide wire and the second guide wire such that the first guide wire and the second guide wire are capable of protruding from the first distal- end opening and the second distal-end opening, respectively,