An endovascular treatment aiding device whose filter is not caught by a treatment device during retrieval of the endovascular treatment aiding device is disclosed. The treatment aiding device comprises: a flexible shaft； a filter fixed to the shaft, which can be opened and closed in an umbrella-like manner； linear supporting members each of which connects the end portion in the opening-section side of the filter to a part of the shaft such that the filter can be closed by applying an external force to the proximal side； an elastomer member composed of an elastomer in which a penetrating hole (A) 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 (A), wherein the outer diameter of the end face in which the penetrating hole (A) 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 (B) 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 (B) 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.
