A device for mounting on the insertion section of an insertion device is provided with: annular members (56, 57) which are affixed to the outer periphery of the insertion section (2); a compression section (52) which is provided movable forward and backward in the longitudinal direction E within a range E1 restricted by the annular members (56, 57), can be expanded and contracted within a subject by operation from the outside of the subject, and is engaged with an inner wall of the subject after being expanded; and a pivot restriction section (52at, 55) which, on the outer periphery of the insertion section (2), restricts the angle of pivot of the compression section (52) in the outer peripheral direction C.