A treatment aiding device includes a flexible shaft; a filter; linear supporting members; an elastomer member composed of an elastomer in which a penetrating hole is formed, which elastomer member is arranged in the proximal side relative to the opening section of the filter such that the shaft penetrates the penetrating hole, wherein the outer diameter of the end face in which the penetrating hole is formed is larger than the outer diameter of the opening-section side of the filter when the filter is closed; and an outer tube in which a penetrating hole is formed, which outer tube is arranged in the proximal side relative to the opening section of the filter such that the shaft penetrates the penetrating hole in a state which allows movement of the shaft in the longitudinal direction; wherein the elastomer member is fixed in the distal side of the outer tube.