An outer port sheathing an overtube, includes: a first cylindrical member having a distal end opening from which the overtube is delivered; a second cylindrical member that is rotatably connected to the first cylindrical member and has a base end opening into which the overtube is introduced; rotation restriction means that restricts rotation of the overtube with respect to the second cylindrical member; a spring member that is deformable between a rotation locked state where rotation of the second cylindrical member with respect to the first cylindrical member is restricted by engagement of the spring member with the first cylindrical member, and a rotation unlocked state where the engagement is released and the rotation is allowed; and a rotation operating member that is rotatable in an axial direction of the second cylindrical member and deforms the spring member between the rotation locked state and the rotation unlocked state.